Allows group_signifigance to skip rows

if a group row would cause an error in kruskal wallis, the p_val and
test stat are returned as nan.

Addresses issue #1721 
@JWDebelius, can you specify the specific error type(s) that you expect here? It's generally not a good idea to catch all exceptions ([here](http://stackoverflow.com/questions/10594113/bad-idea-to-catch-all-exceptions-in-python) is a decent discussion of why that is).