Figure out how to handle grad errors

[baileyparker]

No description provided.

[baileyparker]

Currently the test assertion is that if the phase file has a line starting with error: , we expect stderr to start with error: as well. This allows grads to continue emitting errors without the test harness complaining that stderr doesn't exactly match. Ideally, we should be able to assert that in the grad case the correct number of errors are omitted. Additionally, it would be a nicety if we could have some sort of fuzzy matching on location (although this might be tricky, because I'm not convinced that every error has a only one valid location that can be reported).