Handle exceptions on create_dhcp_port

Handle exceptions on create_dhcp_port

If a network/subnet is deleted while creating the dhcp
port, the agent will detect a conflict on state of the
network and deal with it accordingly.

A concurrent delete may manifest itself via a number
of exceptions, IPAddressGenerationFailure amongst others,
hence the refactoring of the error handling logic into its
own utility method.

Partial-bug: #1253344
Related-bug: #1243726

Change-Id: I442beb5f82f3db8786eea53926903ef0ba0efbf1