Show entire error message

[rfindler]

This is intended to improve the error messages when rackunit catches an exception and decides that a test failed because of the caught exception.

Before this commit, rackunit would show only the basic message in the exception. Afterwards, it uses the error display handler, which can include a bunch more useful information (notably stack traces and information about syntax errors)

[rfindler]

I see that this is causing some test cases to fail, probably because they're checking the precise content of the error messages so probably the expected results need to be audited and adjusted.