Improve error handling for DR_task.create.
Improve error handling for DR_task.create.

The PBD.unplugs in the "with" block shouldn't throw errors since all
PBDs created for the SR by the time the error occurs with be either PBDs
which plugged cleanly, or a PBD which threw an exception on plug and so
isn't attached.

Signed-off-by: John Else <37b74501daa612e48fea94f9846ed4ddeb6a8f07@citrix.com>

johnelse/xcp-rrdd,koushikcgit/xcp-rrdd,djs55/xcp-rrdd,koushikcgit/xcp-rrdd,simonjbeaumont/xcp-rrdd,johnelse/xcp-rrdd,robhoes/squeezed,sharady/xcp-networkd,djs55/xcp-rrdd,djs55/squeezed,djs55/xcp-networkd,simonjbeaumont/xcp-rrdd,djs55/xcp-networkd,koushikcgit/xcp-rrdd,koushikcgit/xcp-networkd,sharady/xcp-networkd,koushikcgit/xcp-networkd