Throws errors if the font isn't found

[tmcw]

fontnik.load will throw an error if the given font can't be found or loaded. Instead, it should call the callback with an error argument.

[mikemorris]

If a font can't be found or loaded, the error should be handled externally, in an fs.readFile callback for example, before passing a buffer on to fontnik.load.

The callback is currently called with an error argument if fontnik.load fails because of an invalid font buffer (a malformed upload or simply new Buffer(0) for example).