[lib] Avoid infinite loop on 500 errors in file chooser popup
[lib] Avoid infinite loop on 500 errors in file chooser popup

Traceback (most recent call last):
  File "/usr/share/hue/build/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/share/hue/build/env/lib/python3.6/site-packages/django/utils/deprecation.py", line 114, in __call__
    response = response or self.get_response(request)
  File "/usr/share/hue/build/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 49, in inner
    response = response_for_exception(request, exc)
  File "/usr/share/hue/build/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 103, in response_for_exception
    response = handle_uncaught_exception(request, get_resolver(get_urlconf()), sys.exc_info())
  File "/usr/share/hue/build/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 142, in handle_uncaught_exception
    return callback(request, **param_dict)
  File "/usr/share/hue/desktop/core/src/desktop/views.py", line 454, in serve_500_error
    return render("500.mako", request, {'traceback': traceback.extract_tb(exc_info[2])})
  File "/usr/share/hue/desktop/core/src/desktop/lib/django_util.py", line 239, in render
    return render_json(data, request.GET.get("callback"), status=status)
  File "/usr/share/hue/desktop/core/src/desktop/lib/django_util.py", line 319, in render_json
    json = encode_json(data, indent)
  File "/usr/share/hue/desktop/core/src/desktop/lib/django_util.py", line 291, in encode_json
    return json.dumps(data, indent=indent, cls=Encoder)
  File "/usr/lib/python3.6/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/lib/python3.6/json/encoder.py", line 201, in encode
    chunks = list(chunks)
  File "/usr/lib/python3.6/json/encoder.py", line 430, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/lib/python3.6/json/encoder.py", line 404, in _iterencode_dict
    yield from chunks
  File "/usr/lib/python3.6/json/encoder.py", line 325, in _iterencode_list
    yield from chunks
  File "/usr/lib/python3.6/json/encoder.py", line 437, in _iterencode
    o = _default(o)
  File "/usr/share/hue/desktop/core/src/desktop/lib/django_util.py", line 89, in default
    return json.JSONEncoder.default(self, o)
  File "/usr/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'FrameSummary' is not JSON serializable

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