Fix for Issue #58 - thrown error is reported, and can be compared to an expected value
reset the error message between each run
Encapsulate the expected/actual messages in curly brackets, as with other expected/actual comparisons
Improve exception reporting by indicating whether an error was raised or not independently of whether the error message matched
rejigged tests to handle error code matching fixes #58

* This new behavior sort of affected previous tests. Fixed those up and
added a helper function to cleanup the PesterThrow function.
Merge branch 'edharper01-issue58'