Split font-pack-config into fonts-with-default.js and fonts-without-default.js to prepare for additional checks and easier configuration.
Add font checks and fix locale_to_subdirs processing.

Added font checks:

* Either a `default` directory or `default` override in locale_to_subdirs must be specified
* If a font file does not exist

Fixed locale_to_subdirs processing:

* A locale specified in locale_to_subdirs now shows up in the font configuration for node-font-face-generator.