Improve handling of errors in ScalaCheck properties.

Before this commit, exceptions thrown in property evaluation would
only show the class and description, but no stack trace. This code
updates the `PropertyMatcher` classes to throw an `AsertionError`
containing the original exception and pretty description of the
property evaluation result if an exception is thrown in either the
property or the generator supplying it.
Don’t return Nil
Add some broken tests
Fix tests.