Raise better exception if duplicate security groups

Previously a 500 error was raised if one was trying to launch an instance
with a security group that shared the same name as another security group.
With this patch a more clear error message is raised.

Closes-bug: #1161472

Change-Id: I285bedbb2e0b3f4cd24cfa3a9b17131ad0200afa
Merge "Raise better exception if duplicate security groups"