Non-zero error code on fail

[Dabsunter]

Currently, when the xunit-viewer command fail (for any reason) the tool still return the 0 error code on exit (default), which makes it non trivial to detect error in non-JS scripts.

Ideally, xunit-viewer could associate specific error code to common issues, but guaranteeing that a 0 error code is associated to a success state, and non-zero otherwise would be a good start.