Pass the extension driver exception to plugin

Pass the extension driver exception to plugin

The extension driver is intercepted by driver manager currently. It will
cover the errors/exceptions happened in extension drivers. The extension
process will continue even if preceding extension driver get a wrong/useless
extended result, or even no result.
This patch make process_[create|update]_<resource>() and
extend_<resource>_dict() methods return the exception, and log it with proper
level respectively, and also include a minor optimization for the
extend_<resource>_dict() methods.

Change-Id: I20a249c47b58292125476bc44b2372ca959509e3
Closes-Bug: #1468990
(cherry picked from commit 73845d564c910bb9113a3ba5963b368384efbaad)