Minor robustness improvements for isolationtester.

Notice and complain about PQcancel() failures.  Also, don't dump core if
an error PGresult doesn't contain severity and message subfields, as it
might not if it was generated by libpq itself.  (We have a longstanding
TODO item to improve that, but in the meantime isolationtester had better
cope.)

I tripped across the latter item while investigating a trouble report on
buildfarm member spoonbill.  As for the former, there's no evidence that
PQcancel failure is actually involved in spoonbill's problem, but it still
seems like a bad idea to ignore an error return code.