Make AddExistingSubkeyJob reject subkeys with combined algorithms
* src/qgpgmeaddexistingsubkeyjob.cpp (add_subkey): Return error if subkey has more than one keygrip.
Subkeys using combined algorithms have more than one keygrip. The
AddExistingSubkeyJob job would fail for such subkeys with an unknown
error. Fails with a not-supported error instead.
- GnuPG-bug-id: T7883