Pass instance to _error_out_instance_on_exception

This is part of a series of patches to eventually get instance objects
passed to _instance_update so that we don't have to make a separate call
to update the instance via conductor when using an object already.  We
start with _error_out_instance_on_exception, then need to handle
_set_instance_error_state and then will change _instance_update.

Adds unit tests for the three different exception blocks handled
in the context.

Part of blueprint compute-manager-objects-juno

Change-Id: Ib3e64089a45cfab37e9c3570b94fed59b0d8f22c
Merge "Pass instance to _error_out_instance_on_exception"