cxnmgr error handling

Reviewer: @rlane

On error notification, check socket error value first before disconnecting.

After interrupting the flow stats output from the 'show flowtable' cli, connections to 127.0.0.1:6634 are dropped after the handshake completes:
Aug 20 17:14:32 lb9-4-fixme ofad[28657]: ofconnectionmanager: TRACE: Soc 18 ready, cxn 0x102fffd0. rd 1. wr 0. er 1
Aug 20 17:14:32 lb9-4-fixme ofad[28657]: ofconnectionmanager: TRACE: cxn 127.0.0.1:6634: Error reading from socket: Resource temporarily unavailable
Aug 20 17:14:32 lb9-4-fixme ofad[28657]: ofconnectionmanager: TRACE: cxn 127.0.0.1:6634: Still need 8 bytes for msg hdr
Aug 20 17:14:32 lb9-4-fixme ofad[28657]: ofconnectionmanager: ERROR: Error seen on connection 127.0.0.1:6634: Success
Aug 20 17:14:32 lb9-4-fixme ofad[28657]: ofconnectionmanager: VERBOSE: cxn 127.0.0.1:6634: Forcing disconnection

It's not clear to me how to clear the POLLERR indication from poll(); any enlightenment would be appreciated.  Thanks.