Slightly Prettier Stack Trace Printing on Exception

Closes #14 

Moves exception error handling to events to keep `AbstractTestCase` uncluttered.

The only thing I'm a bit iffy about is where to "draw the line" for stack traces. Right now it's printing everything, so you'll end up seeing the stack trace all the way back to the `demeanor` script. I like the transparency of this, but I'm not sure it's great for end users.