Raise correct exception when validate_connector failed

Raise correct exception when validate_connector failed

Cinder volume manager uses validate_connector() method to verify if required
information is in connector when handling initialize_connection() request.
validate_connector() is actually a pure input validation method, basically
checking if 'initiator' or 'wwpns' is in connector if storage protocol is
iSCSI or FC.  However, when required information is missing, currently drivers
raises either VolumeBackendAPIException or VolumeDriverException, which would
then bubble up to API and then to user (Nova) as InternalServerError.

This change adds a new exception - InvalidConnectorException, that drivers
should raise when connector is found not valid.  With that, Cinder API would
raise BadRequest instead to user, suggesting things are missing in request.

Change-Id: I4f04f5d0c558404836a2bd270f7f22f3f2d4f314
Closes-bug: #1409580