Correct error message from generate-info.js

This provides a better error message if there is a failure walking the externs dir.

@ahocevar I came across this minor issue when looking into reworking this task for Windows support.  Curious why `externs/geojson.js` gets special treatment here.  Would things work if we treated all `externs/*.js` equally (meaning we pass all to the jsdoc task if any are newer than the `info.json` file)?