Catch vol not found in SolidFire transfer

The accept_transfer call in SolidFire wasn't catching
the volume_not found exception like it should be (and
like other methods do).

This patch just adds a check for volume is None on the
get_volume call during accept_transfer and raises an
appropriate exception if we run into that case.

While we're at it, add some more detail to the "NotFound"
error messages to include the driver operation as per
request of reviewers.

Change-Id: Ie6b56953028865eb06ebdaae3416339b8a371351
Closes-Bug: #1357261
Merge "Catch vol not found in SolidFire transfer"