Multi line exception message hidden by Python unittest runner

TestSlide exceptions often have a multi-line message. While TestSlide properly shows them in full, Python unittest test runner seem to swallow all but the first exception line. This makes it difficult for users to understand why things fail.

This can potentially be fixed by detecting when **not** running with TestSlide test runner (or when running with Python unttest test runner) and change all exceptions to be single line.

CC @pallotron