Issue 846388. Adds a call to PyErr_CheckSignals to

Issue 846388. Adds a call to PyErr_CheckSignals to
SRE_MATCH so that signal handlers can be invoked during
long regular expression matches.  It also adds a new
error return value indicating that an exception
occurred in a signal handler during the match, allowing
exceptions in the signal handler to propagate up to the
main loop.  Thanks Josh Hoyt and Ralf Schmitt.