Oliver/HFX-908:SCTX-1334: Add exception handlers

In response to code review with djs55

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>

Conflicts:

	ocaml/xapi/message_forwarding.ml
Raise the correct error if a xenops task is cancelled

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>