PR30442: failing optional statement probes should not trigger pass2 exceptions

PR30442: failing optional statement probes should not trigger pass2 exceptions

In tapsets.cxx, query_cu() and query_module() aggressively caught &
sess-print_error'd semantic_errors from subsidiary call sites.  They
are unaware of whether the probe in question is being resolved within
an optional (? or !) context.  Instead of this, they now simply let
the exceptions propagate out to derive_probes() or similar, which does
know whether exceptions are errors in that context.  That means
exceptions can propagate through elfutils iteration machinery too,
perhaps risking C level memory leaks, but so be it.

This fix goes well beyond statement probes per se, but hand-testing
and the testsuite appear not to show regressions related to this.

Added semok/badstmt.exp to test.