Changes nec-agent to catch RPC exceptions in the main
daemon loop. It fixes the following two bugs.
- nec-agent dies with RPC timeout in secgroup RPC
(Closes-Bug: #1235106)
- port_added message can be dropped when RPC timeout occurs
(Closes-Bug: #1235111). Moves exception catch from PluginApi
to the main daemon loop.
Change-Id: I12394b739fc35bff271543f4625c5e3152e534ae