Error message improvements

[DatGuyChucky]

Summary

Improved error messages for CheckFailure, Forbidden, MissingRequiredArgument, and cog specific errors in verify and RFR

close #84

Checklist

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

• Have you tested the changes? (pytest & dpytest)
• Have you followed PEP-8 for naming and styling?
• Has your code been properly documented with RestructuredText docstrings?
• Have you added your changes to CHANGELOG.md under the [Unreleased] heading?
• If your code added new bot commands, have you updated documentation.json?

• All of this code is your own, or follows the author repo's licence.

[codecov]

Codecov Report

Merging #321 (3aee502) into master (9150cd8) will decrease coverage by 0.04%.
The diff coverage is 76.66%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
- Coverage   86.72%   86.69%   -0.04%     
==========================================
  Files         111      111              
  Lines        8161     8172      +11     
==========================================
+ Hits         7078     7085       +7     
- Misses       1083     1087       +4     

Flag	Coverage Δ
unittests	86.69% <76.66%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
koala/cogs/verification/cog.py	70.28% <50.00%> (-0.46%)	⬇️
koala/cogs/react_for_role/cog.py	49.09% <66.66%> (+<0.01%)	⬆️
koala/env.py	81.25% <81.25%> (ø)
tests/test_koalabot.py	100.00% <100.00%> (ø)

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[JayDwee]

unit tests need to be added to allow this to pass CodeCov check