Correct error message from generate-info.js

[tschaub]

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)?

[ahocevar]

This change looks good, please merge.

The reason why externs/geojson.js gets special treatment is because it is the only extern that provides types that we expose to the API. I restricted jsdoc generation for externs to olx.js and geojson.js because externs/webgl-debug.js has annotations that cause the jsdoc task to fail, like this one here:

ERROR: The @description tag requires a value. File: webgl-debug.js, line: 31

If the whole externs directory is included, these annotations need to be fixed first. The resulting externs file would be the same, so it might make sense to go down that route, resulting in a more straightforward generate-info task. The only downside is that whenever we add externs, we'll have to make sure that all jsdoc annotations are valid.