Allow AgentExceptions to be logged properly

Remove the special handling in AgentException so that
the default Exception behavior is used when logging, and
details are shown.

Partially implements blueprint improve-engine-logging
Closes-Bug: #1316360
Change-Id: I9730c3e0733f1d6e7d1e40a03aea8499c9605093
Merge "Allow AgentExceptions to be logged properly"