Specific User-Defined Error Messages on Test Case Failure

[Jmdwow]

Is your feature request related to a problem? Please describe.
When a test case fails currently, the reason for the assertion failure is printed in full, along with other potentially confusing information for students.

Describe the solution you'd like
It would be great to have a way to specify an error message for each assert, and have that error message be the only thing that gets output if the assert fails.

Describe alternatives you've considered
A function-wide error message decorator could work, but would lack the nuance of being able to give the student what their output was, or having different messages for asserts for different incorrect outputs.

[thoward27]

I think this is going to be a tricky one to pull off cleanly. I see a lot more value in error messages for each assert than an error message for the function overall.

[Jmdwow]

Same. And maybe an option to specifically print what's on stderr when the assert fails might be good. (For compile errors, etc)

[thoward27]

We should consider a two-fold approach. We could spin up two new features that could combine to create this effect.

1. Implement additive messages, so that people can add commentary to the end of the normal output. (I believe this is already done by unittest, but we should make sure grade captures it.)
2. A decorator which disables tracebacks and only prints any provided messages.

@tracebacks(False)
def test_something():
    self.assertTrue(False, "Your bool was bad.")
    return

Would then produce only "Your bool was bad"

[Jmdwow]

Yeah, that sounds good