This operation returns a subnetwork given a subnetwork name.

Operation Exceptions:
1. INTERNAL_ERROR - Raised in case of non-specific OS internal failure.
2. NOT_IMPLEMENTED - Raised when the OS does not support this operation.
3. INVALID_INPUT - Raised when subnetName does not reference a multiLayerSubnetwork object.
4. ENTITY_NOT_FOUND - Raised when subnetName references a multiLayerSubnetwork object that does
    not exist.
