handle mocha errors correctly in grunt: catch exceptions , pass correct ...

This fixes the following behaviour:
- when you have multiple defined webdriver grunt definitions, allow mocha to be run in a clean slate on consequent runs. (clear require cache) ; otherwise the second run will not execute any tests
- catch mocha exceptions correctly (bypassing the grunt unhandled Exception handler) , and report the correct number of errors. Otherwise it will stop after a failing test.

Solves GH-3 issue.

Would appreciate a new release of the package if possible.