Implement result-type restriction in ganeti-noded

Since all rpc calls were converted, we can now:
  - enforce result type to (status, data)
  - convert all unhandled exceptions to (False, str(err))

This makes sure that all unhandled errors are reported to rpc users.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>