i18n: Improve error logging when language JSON read fails
i18n: Improve error logging when language JSON read fails

Before it only logged an error like this:

    SyntaxError: Unexpected string in JSON at position XYZ

Now it also logs the filename, making it easier to figure out where
the bad data is:

    failed to read file /path/to/etherpad-lite/src/locales/en.json: SyntaxError: Unexpected string in JSON at position XYZ

storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite