Don't log logexpect error level from within the callback routine

vtc_log does a pthread_exit when reporting an error condition. When
this happened from within the API callback function for logexpect, the
API internal data structures were left in an inconsistent state,
causing asserts do be thrown during VSLQ_Delete().

Signal the failed expect back from the callback function and do the
vtc_log error level after the API function has returned.