Better json schema loading error message

When json schema file is malformed, it will print following stacktrace:

Traceback (most recent call last):
  File "/usr/bin/foris-controller", line 8, in <module>
    sys.exit(main())
  File "foris-controller/foris_controller/controller/__main__.py", line 229, in main
    prepare_app_modules(BaseOpenwrtHandler, [e[0] for e in options.extra_module_path])
  File "foris-controller/foris_controller/app.py", line 143, in prepare_app_modules
    app_info["validator"] = ForisValidator(schema_dirs, definition_dirs)
  File "/usr/lib64/python3.8/site-packages/foris_schema/validator.py", line 215, in __init__
    schema = json.load(f)
  File "/usr/lib64/python3.8/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.8/json/decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)
json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 668 column 17 (char 26397)

Meaning that json schema file is malformed, which it isn't really obvious at first glance.

It would be nice to have some error message instead of this stacktrace, for example:

Failed to load json schema 'mymodule.json': json.decoder.JSONDecodeError: Expecting property name enclosed in double quotes: line 668 column 17 (char 26397)