Printing outcomes buffer reveals too much information

[adhameer]

This is in relation to the merged PR #250, which added the outcomes buffer to the list of buffers to be printed when there is an error grading an exercise.

This is very useful for debugging, but when this buffer is printed on an instance of learn-ocaml that is exposed to students, it reveals far too much information: in particular it reveals the entire signature of the test.ml file for that exercise.

It would be useful to have this feature controlled by some sort of debug flag, and ideally to hide all this information about test.ml but still show the student some useful error message in the case of an error while grading their solution.