Fix bug in profiler modifications detected only in debug builds.

The new profiler event stream includes a "return" event even when an
exception is being propogated, but the machinery that called the profile
hook did not save & restore the exception.  In debug mode, the exception
was detected during the execution of the profile callback, which did not
have the proper internal flags set for the exception.  Saving & restoring
the exception state solves the problem.