Make `bundler.sh` handle JSON file argument; Improve error handling/behaviour

#### Handle JSON files

`bundler.sh` now correctly handles a JSON file if passed on in the arguments.

#### Improve error handling/error messages:

- Return appropriate exit code from `__loadAsset`.
- Pass through exit code of `__loadAsset` as script exit status in `alfred.bundler.misc.sh` (Similar changes are presumably necessary in `alfred.bundler.sh`, but I haven't made them).
- Return specific error message if the JSON file isn't found.