improve error message site not specified

[metofficedera]

Closes #217 .

Improves the error message when SITE is not set. The message now tells users to specify a site using '-O ' and gives
'cylc vip -O metoffice' as an example.

PR creation checklist for the developer

• The <issue_number> above ☝️ has been replaced with the issue number.
• main has been selected as the base branch.
• The feature branch name follows the format <issue_number>_<short_description_of_feature>.
• The text of the PR title exactly matches with the text (not including the issue number) of the issue title.
• Appropriate reviewers have been added to the PR (once it is ready for review).
• The PR has been assigned to the developer(s).
• The same labels as on the issue (except for the good first issue label) have been added to the PR.
• The Climate Model Evaluation Workflow (CMEW) project has been added to the PR.
• The appropriate milestone has been added to the PR.

Definition of Done for the developer

• This PR contains only the changes needed to meet all acceptance criteria.
• The change in this PR follows the requirements in the wiki: Developer Guide (including copyrights).
• The GitHub Actions workflow checks pass.
• The tests run locally and pass (Note: the tests are not run by the GitHub Actions workflow, see wiki: Run the tests locally).
• Updating the Rose metadata (select one of the following):
  • Rose metadata related to the change has been added or updated.
  • The change does not require Rose metadata to be added or updated.
• Rendering the Rose metadata (select one of the following):
  • The Rose GUI shows the change as expected.
  • The change in this PR does not affect the Rose GUI.
• Updating the tests (select one of the following):
  • Tests related to the change have been added or updated.
  • The change does not require tests to be added or updated.
• Updating the user documentation (i.e. everything in the doc directory, including the Quick Start section; select one of the following):
  • The user documentation related to the change has been updated appropriately.
  • The change in this PR does not require the user documentation to be updated.
• Rendering the user documentation (wiki: Build the documentation locally provides instructions; select one of the following):
  • The HTML pages show the change as expected.
  • The change in this PR does not affect the HTML pages.
• Updating the API documentation (e.g. docstrings in Python modules; select one of the following):
  • The API documentation related to the change has been updated appropriately.
  • The change in this PR does not affect the API documentation.

Copyright and IPR:

	GitHub handles
I confirm that all code is my own and that my contributions are not subject to copyright or license restrictions.	GitHub handle(s) of all developers of this PR
I confirm I have not knowingly violated intellectual property rights (IPR) and have taken sensible measures to prevent doing so, including appropriate attribution for usage of Generative AI. I confirm that this work is my own, and I understand that it is my responsibility to ensure I am not violating others’ IPR. This includes taking reasonable steps to ensure that all tools used while creating this contribution did not infringe IPR.	GitHub handle(s) of all developers of this PR

PR creation checklist for the reviewer

• The <issue_number> above ☝️ has been replaced with the issue number.
• main has been selected as the base branch.
• The feature branch name follows the format <issue_number>_<short_description_of_feature>.
• The text of the PR title exactly matches with the text (not including the issue number) of the issue title.
• Appropriate reviewers have been added to the PR (once it is ready for review).
• The PR has been assigned to the developer(s).
• The same labels as on the issue (except for the good first issue label) have been added to the PR.
• The Climate Model Evaluation Workflow (CMEW) project has been added to the PR.
• The appropriate milestone has been added to the PR.

Definition of Done for the reviewer

• This PR contains only the changes needed to meet all acceptance criteria.
• The change in this PR follows the requirements in the wiki: Developer Guide (including copyrights).
• All developers of this PR have added their names to the Copyright and IPR statements above.
• The GitHub Actions workflow checks pass.
• The tests run locally and pass (Note: the tests are not run by the GitHub Actions workflow, see wiki: Run the tests locally).
• Updating the Rose metadata (select one of the following):
  • Rose metadata related to the change has been added or updated.
  • The change does not require Rose metadata to be added or updated.
• Rendering the Rose metadata (select one of the following):
  • The Rose GUI shows the change as expected.
  • The change in this PR does not affect the Rose GUI.
• Updating the tests (select one of the following):
  • Tests related to the change have been added or updated.
  • The change does not require tests to be added or updated.
• Updating the user documentation (i.e. everything in the doc directory, including the Quick Start section; select one of the following):
  • The user documentation related to the change has been updated appropriately.
  • The change in this PR does not require the user documentation to be updated.
• Rendering the user documentation (wiki: Build the documentation locally provides instructions; select one of the following):
  • The HTML pages show the change as expected.
  • The change in this PR does not affect the HTML pages.
• Updating the API documentation (e.g. docstrings in Python modules; select one of the following):
  • The API documentation related to the change has been updated appropriately.
  • The change in this PR does not affect the API documentation.

Important

• Remember to re-check the Definition of Done after making changes in response to a review.
• The developer merges the PR.
• Remember to use the format #<pull_request_number>: <pull_request_title> when writing the merge commit message for the pull request, so the pull request number is immediately visible on GitHub, regardless of the length of the pull request title.