logging an optional input location

[Dan Eble]

There’s this repetition in context.cc:

  if (origin)
    {
      origin->warning (_f ("cannot find or create `%s' called `%s'",
                           ly_symbol2string (n).c_str (), id));
    }
  else
    {
      warning (_f ("cannot find or create `%s' called `%s'",
                   ly_symbol2string (n).c_str (), id));
    }

I put up with this when it was just one instance, but it’s starting to multiply 
in my work in progress.

I see that ::warning already accepts a location string as an optional second 
parameter, so I am thinking of using that rather than creating yet another 
interface.  I am thinking of defining a helper function message_location(const 
Input*) to be used like this:

  warning (_f (. . .), message_location (origin));

Are there any strong objections to this?  If so, what would you prefer?
— 
Dan