Pass back query errors as 400s

[jrdh]

When a user makes a query using the elasticsearch raw endpoint they can send crap queries and they should have a useful error return if they do a duff one. For example:

{
  "query": {
    "exists": "weight"
  }
}

produces this error in the logs:

[Tue Jul 02 17:06:48.025699 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/src/ckanext-versioned-datastore/ckanext/versioned_datastore/logic/action.py", line 643, in datastore_search_raw
[Tue Jul 02 17:06:48.025717 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     search = Search.from_dict(search)
[Tue Jul 02 17:06:48.025725 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/search.py", line 375, in from_dict
[Tue Jul 02 17:06:48.025755 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     s.update_from_dict(d)
[Tue Jul 02 17:06:48.025763 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/search.py", line 417, in update_from_dict
[Tue Jul 02 17:06:48.025774 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     self.query._proxied = Q(d.pop('query'))
[Tue Jul 02 17:06:48.025784 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/query.py", line 18, in Q
[Tue Jul 02 17:06:48.025793 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     return Query.get_dsl_class(name)(_expand__to_dot=False, **params)
[Tue Jul 02 17:06:48.025798 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/utils.py", line 224, in __init__
[Tue Jul 02 17:06:48.025807 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     self._setattr(pname, pvalue)
[Tue Jul 02 17:06:48.025813 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/utils.py", line 263, in _setattr
[Tue Jul 02 17:06:48.025820 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     value = list(map(shortcut, value))
[Tue Jul 02 17:06:48.025826 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]   File "/usr/lib/ckan/default/lib/python2.7/site-packages/elasticsearch_dsl/query.py", line 18, in Q
[Tue Jul 02 17:06:48.025833 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938]     return Query.get_dsl_class(name)(_expand__to_dot=False, **params)
[Tue Jul 02 17:06:48.025849 2019] [:error] [pid 45102:tid 140182646204160] [remote 157.140.2.190:20938] TypeError: DslMeta object argument after ** must be a mapping, not unicode

and therefore the user gets a 500 error when they should get a 400 error (the exists query uses a "field" key).