autocomplete service returns 500 internal server error when given indexes that do not exist

[FrankHassanabad]

Describe the bug:
When you hit the autocomplete service within Kibana using either kibana.yml strategies of:

data.autocomplete.valueSuggestions.method: 'terms_agg'

or the newer default one (from #100174)

data.autocomplete.valueSuggestions.method: 'terms_enum'

You get back a 500 internal service error:

server   error  [13:55:37.767]  Error: Internal Server Error
    at HapiResponseAdapter.toInternalError (/Users/frankhassanabad/projects/kibana/src/core/server/http/router/response_adapter.ts:53:19)
    at Router.handle (/Users/frankhassanabad/projects/kibana/src/core/server/http/router/router.ts:287:34)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at handler (/Users/frankhassanabad/projects/kibana/src/core/server/http/router/router.ts:228:11)
    at exports.Manager.execute (/Users/frankhassanabad/projects/kibana/node_modules/@hapi/hapi/lib/toolkit.js:60:28)
    at Object.internals.handler (/Users/frankhassanabad/projects/kibana/node_modules/@hapi/hapi/lib/handler.js:46:20)
    at exports.execute (/Users/frankhassanabad/projects/kibana/node_modules/@hapi/hapi/lib/handler.js:31:20)
    at Request._lifecycle (/Users/frankhassanabad/projects/kibana/node_modules/@hapi/hapi/lib/request.js:370:32)
    at Request._execute (/Users/frankhassanabad/projects/kibana/node_modules/@hapi/hapi/lib/request.js:279:9)

Steps to reproduce:
Use a CURL script to hit the service with a non-existent index such as:

# Blows up with 500 error if given nonexistent index
curl -s -k \
  -H 'Content-Type: application/json' \
  -H 'kbn-xsrf: 123' \
  -u ${ELASTICSEARCH_USERNAME}:${ELASTICSEARCH_PASSWORD} \
  -X POST ${KIBANA_URL}/api/kibana/suggestions/values/index-does-not-exist \
  --data-raw '{"query":"","field":"host.name","filters":[]}'

# Error returned
# {"statusCode":500,"error":"Internal Server Error","message":"An internal server error occurred."}%

Expected behavior:
Errors from Elasticsearch such as 404 not found would be bubbled up as nice REST API semantics of a wrapped 404 error and that REST services do not return 500 internal server errors.

Kibana version:
7.14.0 BC-1, 8.0.0

Elasticsearch version:
7.14.0 BC-1, 8.0.0

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)