Verify the files that are bundled together exist.

I believe that the code for this gulp extension was written using `gulp.src` for simplicity. While that works, `gulp.src` does glob matches, so `gulp-bundle` isn't emitting any error messages.

In working with the source, I realized that with this extension, you can use globs in your build blocks, so I went with that assumption. It's a nice feature of the project, but I don't know if it was intended.

Anyway, this will log error messages when files aren't found. It doesn't make it a hard error, though, so the task continues and concatenates whatever it did find (which is the current behavior).