Raise sane error when failing to modify security group

Horizon catches and propagates recoverable exceptions back to the
user.  This code previously caught all exceptions and then raised
Exception (with an unhelpful message), which Horizon propagated as an
even less helpful message.  By raising the error as a
novaclient.exception.ClientException, we give Horizon a fighting
chance of passing a useful (but sanitized) error message to the user.

This changes the message the user sees from:

Error: There was an error submitting the form. Please try again.

To:

Error: Unable to modify instance "test10".
Error: Failed to modify 1 instance security groups

The error condition can be simulated by trying to modify security
groups on a suspended instance.

Closes-Bug: #1329888
Change-Id: I38d9977d6d20dfeddbb77b1a7e63e74597bf7331
Merge "Raise sane error when failing to modify security group"