Defensive coding if exception data is wonky
Defensive coding if exception data is wonky

The xunit runner utility can produce false positives when parsing
multiple nested exceptions. Fixed in 2.1, but this adds some defensive
coding to at least not throw exceptions and fail to finish running the
tests (the message we display might be a bit rubbish though).

Fixes #50

xunit/resharper-xunit,derigel23/resharper-xunit,derigel23/resharper-xunit,xunit/resharper-xunit