#249 Add PHPUnit "diff" output when exceptions are PHPUnit assertions

(it would be better OOP if PHPUnit added this to their exception message, of course)
Remove some now unnecessary PHPUnit exception formatting calls from FeatureContext


I haven't been able to run the full test suite, as the following features segfault for me (Windows 7, PHP 5.3.21 (cli))
 * features\closures\html.feature
 * features\annotations\append_snippets.feature