Don't call preprocess_file() on an empty path if fonts.cfg can't be found

get_wml_location() returns an empty string on failure, and there is
really no point in calling preprocess_file() on that -- it only results
in a confusing error message.

Replaces this sequence often found when Wesnoth can't resolve the path
to its data directory:

  20140612 23:37:05 error filesystem: Trying to open file with empty name.
  20140612 23:37:05 error config: Could not open file
  could not initialize fonts

... with the following one:

  20140612 23:38:52 error font: could not resolve path to fonts.cfg, file not found
  could not initialize fonts

... which should be a little bit more helpful in pointing users in the
right direction in the aforementioned case.
Suggest checking the data dir path when unable to initialize fonts

Not being able to initialize fonts is usually the direct consequence of
a failure to read data/hardwired/fonts.cfg, possibly because the game
data dir was incorrectly set or detected. Thus, it's the ideal point
during early initialization to provide a hint on stderr for players.

I specifically avoided mentioning anything about "installing" the game
so as to not contribute to the common misconception that the game needs
to be installed to run.