passdb: Do not panic in initialize_password_db
A call to initialize_password_db leads to smb_panic in case the backend
returns an error. All callers to initialize_password_db check the return
value, so this code path should return the status instead of calling
smb_panic.
Move the call to smb_panic from pdb_get_methods_reload pdb_get_methods
to get it out of the initialize code path. This allows printing the
proper error message for 'net getlocalsid' which is much nicer than
printing the panic stack trace.