Sync: Improved error message when manifest server RPC call fails

When the RPC call fails, the error message returned by the server
is printed, but it is not obvious that this is caused by RPC call
failure.

Prefix the error message with a descriptive message that explains
what went wrong.

Change-Id: I4b77af22aacc2e9843c4df9d06bf54e41d9692ff

subcmds/sync.py

@@ -566,7 +566,8 @@ later is required to fix a server side protocol bug.
             sys.exit(1)
           self._ReloadManifest(manifest_name)
         else:
-          print('error: %s' % manifest_str, file=sys.stderr)
+          print('error: manifest server RPC call failed: %s' %
+                manifest_str, file=sys.stderr)
           sys.exit(1)
       except (socket.error, IOError, xmlrpc.client.Fault) as e:
         print('error: cannot connect to manifest server %s:\n%s'