Fix: make errors from Presto runner JSON-serializable

[washort]

Errors from Presto can be exceptions raised by requests, which can have exception objects as the message attribute of the exceptions they raise:

TypeError: MaxRetryError("HTTPConnectionPool(host='ec2-45-224-206-100.us-west-2.compute.amazonaws.com', port=8889): Max retries exceeded with url: /v1/statement (Caused by NewConnectionError('<requests.packages.urllib3.connection.HTTPConnection object at 0x7fc4b3ed2c10>: Failed to establish a new connection: [Errno 111] Connection refused',))",) is not JSON serializable
  File "celery/app/trace.py", line 240, in trace_task
    R = retval = fun(*args, **kwargs)
  File "redash/tasks/base.py", line 18, in __call__
    return super(BaseTask, self).__call__(*args, **kwargs)
  File "celery/app/trace.py", line 438, in __protected_call__
    return self.run(*args, **kwargs)
  File "redash/tasks/queries.py", line 470, in execute_query
    return QueryExecutor(self, query, data_source_id, user_id, metadata).run()
  File "redash/tasks/queries.py", line 419, in run
    self.tracker.update(error=error, run_time=run_time, state='saving_results')
  File "redash/tasks/queries.py", line 69, in update
    self.save()
  File "redash/tasks/queries.py", line 59, in save
    connection.set(key_name, utils.json_dumps(self.data))
  File "redash/utils/__init__.py", line 77, in json_dumps
    return json.dumps(data, cls=JSONEncoder)
  File "json/__init__.py", line 251, in dumps
    sort_keys=sort_keys, **kw).encode(obj)
  File "json/encoder.py", line 207, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "json/encoder.py", line 270, in iterencode
    return _iterencode(o, 0)
  File "redash/utils/__init__.py", line 73, in default
    super(JSONEncoder, self).default(o)
  File "json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")

[arikfr]

Thanks!