Fix for silent plugin failures.

This change adds a junitxml functional test that tries to create a xml report in a folder that doesn't exist.
The IO error was not being propagated to the user, so I added a log.exception (that appears in the logging stream in stdout), and a more compact stderr message.

An important side-effect of this change is that now any plugin failure will not stop nose from continuing. I believe this is good as it prevents a bad plugin from breaking the test process, but this could be sometimes bad, in the case of an early failure that should really prevent anything else from happening... that said it looks like I haven't broken anything at this stage.
 
I'm happy to revisit the error messages, or maybe use log.debug('...', exc_info=True) to only display the stacktrace when -v is passed to nose, looking for feedback on this.

This fixes #159.