added some tests for exceptions

I thought I'd spend a few minutes to round out the testing framework a bit to make sure textract is failing in the way it is supposed to. Along the way, I uncovered an unexpected bug that the textract command line tool was exiting with a 0 return code, even when there was an error. Corrected that, but also expect to add a few more tests here. If anyone else has any suggestions, I'm all ears.