Internal Server Error returned for invald input

[bmario]

When for instance the from and to fields are not formatted properly, the metricq code throws an exception, however the server just returns http status code 500 instead of 4xx. We probably should return 400.
For instance:

Error handling request
                                           Traceback (most recent call last):
                                             File "/home/service/envs/grafana/lib/python3.7/site-packages/aiohttp/web_protocol.py", line 418, in start
                                               resp = await task
                                             File "/home/service/envs/grafana/lib/python3.7/site-packages/aiohttp/web_app.py", line 458, in _handle
                                               resp = await handler(request)
                                             File "/home/service/metricq-grafana/metricq_grafana/views.py", line 19, in query
                                               resp = await get_history_data(request.app, req_json)
                                             File "/home/service/metricq-grafana/metricq_grafana/amqp.py", line 30, in get_history_data
                                               start_time = Timestamp.from_iso8601(request["range"]["from"])
                                             File "/home/service/envs/grafana/lib/python3.7/site-packages/metricq/types.py", line 173, in from_iso8601
                                               datetime.datetime.strptime(iso_string, "%Y-%m-%dT%H:%M:%S.%fZ").replace(
                                             File "/home/service/envs/grafana/lib/python3.7/_strptime.py", line 577, in _strptime_datetime
                                               tt, fraction, gmtoff_fraction = _strptime(data_string, format)
                                             File "/home/service/envs/grafana/lib/python3.7/_strptime.py", line 359, in _strptime
                                               (data_string, format))
                                           ValueError: time data 'Fri Aug 14 2020 00:00:00 GMT+0200' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'