Field capabilities API drops shard level exceptions

[cbuescher]

This is a follow up to #68904 where I observed that exceptions thrown on the shard level by the field_caps API are not properly propagated to the caller. Instead we return an empty response. This didn't seem to have been a great issue so far since the biggest error source would have been the only request body parameter, the index_filter section, which typically gets parsed and validated already on the coordinating node and then everything is fine.
With introducing the runtime_mappings section it is now more likely to have errors in there too, but this part of the request doesn't get properly parsed until it reaches the shard (since we need an SearchExecutionContext to parse it). When there are errors here, usually a MapperParserException is thrown, like for the following request:

PUT /my-index/_doc/1
{
  "timestamp": "2012-01-01"
}

POST /my-index/_field_caps?fields=*
{
  "runtime_mappings": {
    "day_of_week": {
      "type": "bad_type",     <---- problem here
      "script": {
        "source": "emit(doc['timestamp'].value.dayOfWeekEnum.getDisplayName(TextStyle.FULL, Locale.ROOT))"
      }
    }
  }
}

Currently we get an empty respone ({ "indices" : [ ], "fields" : { }}) here instead of throwing an error like e.g. in the _search API for the same wrong runtime_mappings definition.

[elasticmachine]

Pinging @elastic/es-search (Team:Search)

[cbuescher]

What I learned from debugging so far:

• we throw a MapperParsingException when parsing the bad section in RuntimeFieldType#parseRuntimeFields
• this exception gets handled and forwarded by TransportResponseHandler#onFailure in TransportFieldCapabilitiesIndexAction which passes it on to TransportFieldCapabilitiesIndexAction#onFailure
• this triggers some tryNext() strategy for iterating shards, but since for this call we have canMatchShard set to false, only the listeners onResponse is called

Even when then listeners onFailure would get called, I guess we still have something missing in the ActionListener in TransportFieldCapabilitiesAction (see TODO here which I guess needs to be adressed to properly pass on errors)