Refactor test_errors.py

[Jasha10]

test_errors should probably be reorganized and broken into smaller functions (while reusing the error validation logic).

Originally posted by @omry in #503 (comment)

[omry]

Please wait with this until we merge #445.

[Jasha10]

What would be a good first step?
Break the list of test cases into several smaller lists?

[omry]

Yeah, potentially calling a common function from various topic specific files.

Keep in mind that for now we have a 2.1 release branch and bug fixes in master are still likely to be ported to that branch. for that reason I think we should wait a while and only do this after Hydra 1.1 is released and had some time to stabilize (otherwise we will need to port the refactoring to the 2.1_branch to avoid conflicts when we cherry-pick other fixes to it).