No indication of soundfont load errors

[newlandsvalley]

If the font loads, we get a callback message with the instrument name and the channel to which it attaches. If it doesn't (usually because the instrument name is mis-typed and thus not available on the Gleitzman site) then an uncaught exception is thrown by the javascript.

We need, ideally, to catch the exception and return it via the PureScript callback.