Avoid spurious warnings in rend_service_introduce

There was some code in the "err:" block that would always log a
warning, reporting an "unknown error" if we hadn't set err_msg.  But
there were also plenty of "goto err" blocks that did their own
logging, and never set err_msg at all.  Now we should only log when
we have an error message to log.

This fixes bug 6638, from no released Tor version.