userdb: return ESRCH if we didn't find a single varlink service

Clearly communicate to callers that we didn't find a single varlink
service, when a lookup is attempted. Note that the fallback's to NSS,
drop-ins and synthesis might eat up this error again, but we should
really make this case reasonably recognizable, in particular as our
various tools already handle this condition correctly and print a nice
message then.

src/shared/userdb.c

@@ -510,7 +510,7 @@ static int userdb_start_query(
         }
 
         if (set_isempty(iterator->links))
-                return ret; /* propagate last error we saw if we couldn't connect to anything. */
+                return ret < 0 ? ret : -ESRCH; /* propagate last error we saw if we couldn't connect to anything. */
 
         /* We connected to some services, in this case, ignore the ones we failed on */
         return 0;