a conjunction of _missing_and _exists_ queries causes an exception

[mhl]

As reported by @symroe, this query produces an error:

• https://yournextmp.popit.mysociety.org/api/v0.1/search/persons?q=_missing_:contact_details.type:twitter%20AND%20_exists_:standing_in.2015.post_id

... which is:

{
  "errors": [
    "Invalid q parameter: _missing_:contact_details.type:twitter AND _exists_:standing_in.2015.post_id",
    "org.elasticsearch.index.query.QueryParsingException: [popit_yournextmp] Failed to parse query [_missing_:contact_details.type:twitter AND _exists_:standing_in.2015.post_id]; org.apache.lucene.queryparser.classic.ParseException: Cannot parse '_missing_:contact_details.type:twitter AND _exists_:standing_in.2015.post_id': Encountered \" \":\" \": \"\" at line 1, column 30.\nWas expecting one of:\n    <EOF> \n    <AND> ...\n    <OR> ...\n    <NOT> ...\n    \"+\" ...\n    \"-\" ...\n    <BAREOPER> ...\n    \"(\" ...\n    \"*\" ...\n    \"^\" ...\n    <QUOTED> ...\n    <TERM> ...\n    <FUZZY_SLOP> ...\n    <PREFIXTERM> ...\n    <WILDTERM> ...\n    <REGEXPTERM> ...\n    \"[\" ...\n    \"{\" ...\n    <NUMBER> ...\n    "
  ]
}