[IMP-dev] exception cleanup

I changed how the check and assert macros work slightly. The former now takes an exception type name as its third argument (instead of an instructor call). The exception is created with the error message passed as the second argument to check (removing the need to specify the message twice). Also, the file and line number are added to the assertion error message.

Finally, by default the error message is not printed, just passed to the exception. Python will print the error message from the exception anyway.