Make linting failure message more clear

Right now when a json file is not linted, the program just says
'Formatted $FILENAME' and exits - this doesn't make it particularly
clear what is wrong when a file is not linted.
This aims to make it more clear.

tools/format/format.cpp

@@ -203,6 +203,7 @@ int main( int argc, char *argv[] )
             std::ofstream fout( filename, std::ios::binary | std::ios::trunc );
             fout << out.str();
             fout.close();
+            std::cout << filename << " needs to be linted" << std::endl;
             std::cout << "Formatted " << filename << std::endl;
             exit( EXIT_FAILURE );
         }