Test failure should cause nonzero exit status

As of 1.0.1, test2junit returns a 0 exit status even when there are test failures. This leads to confusing results in the console output of our CI build. 

I've changed exception handling logic such that test2junit.clj will *rethrow* a caught exception if it contains an exit status, *after* it has attempted to generate the HTML results.

This change also incorporates the change suggested in https://github.com/ruedigergad/test2junit/issues/1