cinder os-detach api returns 500

cinder os-detach api returns 500

If invalid 'attachment_id' is passed to the os-detach
api, then it returns 500 error because even though
VolumeAttachmentNotFound exception is raised from manager,
it gets converted to RemoteError due to rpcapi call.

Caught RemoteError exception in the api and returned 400
error to the user if exception type is VolumeAttachmentNotFound
OR InvalidVolume. In other cases 500 error is raised.

Fixed typo in comment of volume manager.

Closes-Bug: 1449870
Change-Id: I92b03e0af3b7ab517e34edaeffd88f0a00516000