Catch errors thrown by authentication modules.

Authentication modules can throw arbitrary errors - ldap exceptions,
password errors, etc, if auth fails with one of these app specific
errors, the client current dumps its cookies in a stack dump. This
change catches the error cleanly, and transforms it into a handled
authentication exception.

Bugs closed: aurora-259

Reviewed at https://reviews.apache.org/r/19143/
Fixed structdump usage message.

Reviewed at https://reviews.apache.org/r/16062/
s/thermos_observe/thermos_observer/ in README.md

Reviewed at https://reviews.apache.org/r/17293/