Exceptions raised from _prep_block_device not included in instance.fault

Noticed this while working on a patch, when boot-from-volume instance build fails during the _prep_block_device step, the message in the instance.fault looks like this during a 'nova show':

 {"message": "Build of instance ec8f00c3-49a5-4ae0-9c93-1a813b17bf4c aborted: Failure prepping block device.", "code": 500, "created": "2018-12-12T22:06:22Z"}

where the underlying exception raised from _prep_block_device is absent.

We can see the error message being squashed here when BuildAbortException is raised:

https://github.com/openstack/nova/blob/c72dafad8011198b78fd906f2fc7d1126e3f79f3/nova/compute/manager.py#L2341-L2343

We can provide more information for users if we include the exception message from the exception raised in _prep_block_device in the BuildAbortException, which will end up in the instance.fault.