Remove `toString` override from `ErrorPolykey`

[tegefaulkes]

Specification

Recently I added a toString override to the ErrorPolykey class as a way of extending it to include better details about the cause chain. After some discussion about it this needs to be removed infavor of an alternative method.

We need to create a utility what will structure errors as a more informative string to be used when we need to convert errors for use in logger messages. It needs to be aware of the type of error it receives and format it accordingly.

Additional context

• Related: Structured formatting of errors when converted to string js-errors#21

Tasks

1. remove the toString method from ErrorPolykey
2. Create a utility that converts errors into formatted strings.
3. Apply this utility wherever an error is being converted to a string with toString or the String constructor.

[linear]

ENG-437 Remove `toString` override from `ErrorPolykey`