This operation returns a Subnetwork given a subnetwork name.

Operation Exceptions:
1. INTERNAL_ERROR - Raised in case of non-specific EMS internal failure.
2. INVALID_INPUT - Raised when subnetName does not reference a multiLayerSubnetwork object.
3. ENTITY_NOT_FOUND - Raised when subnetName references a multiLayerSubnetwork object that does
    not exist.
