Couple more fixes for sequential forking

After we implemented sequential branches there were 2 problems:
1. CANCELing sequential proxy was falling when it had non-started (yet) branch.
2. Upon sending CANCEL  non-started (yet) branches were kept alive, which used to lead to execution of these branches upon receiving 487 from cancelled branch.

Separate commit: sometimes something fails during processing of SIP messages and by looking into logs it is extremely hard to understand what it is and how severe the failure. So we think it may be beneficial to log actual SIP message causing Exception with the stack trace.