[Ingest Manager] prevent crash on unhandled rejection from setupIngestManager

[jfsiii]

Summary

fixes #73997 (POST /setup with unreachable Registry can crash Kibana)

From comments at the change

  // be sure to return the promise because it has the resolved/rejected status attached to it
  // otherwise, we effectively return success every time even if there are errors
  // because `return undefined` -> `Promise.resolve(undefined)` in an `async` function
  return setupIngestStatus;

master: By returning no value, await setupIngestManager resolves as a success, but hits issues later because of the silent or unhandled failures.

PR: By returning the promise await and KP do what we want

Checklist

Delete any items that are not applicable to this PR.

• Unit or functional tests were updated or added to match the most common scenarios

Testing

Jest test
afd064a to ensure failures in setupIngestManager are treated as errors by await & try/catch

In the terminal
master #73997 shows that POST /setup crashes with a bad registryUrl

this PR, they return a 502 as expected

  if (error instanceof RegistryError) {
    return 502; // Bad Gateway
  }

> curl -X POST 'http://localhost:5601/api/ingest_manager/setup' -H 'kbn-xsrf: <string>' --user elastic:changeme
{"statusCode":502,"error":"Bad Gateway","message":"Error connecting to package registry at http://localhost:9999/search?package=system&internal=true&experimental=true: request to http://localhost:9999/search?package=system&internal=true&experimental=true failed, reason: connect ECONNREFUSED 127.0.0.1:9999"}

> curl -X POST 'http://localhost:5601/api/ingest_manager/setup' -H 'kbn-xsrf: <string>' --user elastic:changeme
{"statusCode":502,"error":"Bad Gateway","message":"Error connecting to package registry at http://localhost:9999/search?package=system&internal=true&experimental=true: request to http://localhost:9999/search?package=system&internal=true&experimental=true failed, reason: connect ECONNREFUSED 127.0.0.1:9999"}

> curl -X POST 'http://localhost:5601/api/ingest_manager/setup' -H 'kbn-xsrf: <string>' --user elastic:changeme
{"statusCode":502,"error":"Bad Gateway","message":"Error connecting to package registry at http://localhost:9999/search?package=system&internal=true&experimental=true: request to http://localhost:9999/search?package=system&internal=true&experimental=true failed, reason: connect ECONNREFUSED 127.0.0.1:9999"}

> curl -X POST 'http://localhost:5601/api/ingest_manager/setup' -H 'kbn-xsrf: <string>' --user elastic:changeme
{"statusCode":502,"error":"Bad Gateway","message":"Error connecting to package registry at http://localhost:9999/search?package=system&internal=true&experimental=true: request to http://localhost:9999/search?package=system&internal=true&experimental=true failed, reason: connect ECONNREFUSED 127.0.0.1:9999"}

> curl -X POST 'http://localhost:5601/api/ingest_manager/setup' -H 'kbn-xsrf: <string>' --user elastic:changeme
{"statusCode":502,"error":"Bad Gateway","message":"Error connecting to package registry at http://localhost:9999/search?package=system&internal=true&experimental=true: request to http://localhost:9999/search?package=system&internal=true&experimental=true failed, reason: connect ECONNREFUSED 127.0.0.1:9999"}

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[neptunian]

tested locally and works

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 777925c

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@elasticmachine merge upstream

[ruflin]

Great to see a test for this!