Expected error tests

We currently have the setup to test that some error occurred during compilation, but we don't test that it's the correct error (other than syntax vs type error) or that it occurs on the correct line.

It'd be nice to be able to annotate source files for testing with expected errors in comments, something like:

x = addInt 1 True -- $expect: Type error: Type mismatch

and then have the setup to verify that that error really occurs there.