import_class() does not report original exception

The utils.import_class() function does a very handy thing, but it catches ImportError and simply raises a NotFound exception without including in the message what the original error was. This caused a problem when glance added xattr to its requirements, in that all we get is this mysterious message that "Client could not be found". It wouldn't take much to augment the "NotFound" message with the text of the original exception, which could give us a better clue as to how to fix the problem.