[8.16] Surface config parsing error under EA managed mode (backport #14574)

[mergify]

Motivation/summary

By wrapping reloader errors in a way that EA manager understands (i.e. in a MultiError of UnitError), newRunner initialization errors, including config parsing errors, will be surfaced and logged.

Checklist

- [ ] Update CHANGELOG.asciidoc I expect this to go into 8.17 first, then 8.16, but there may be uncertainty around this. We can add changelog before the release.

• Documentation has been updated

How to test these changes

Start apm-server in EA managed mode (e.g. on ESS), pass in an invalid integration policy, e.g. setting apm-server.rum.response_headers to a string instead of a list. Check that in fleet UI that EA input unit has failed. Check that an error is logged.

Related issues

Fixes #14560

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This is an automatic backport of pull request #14574 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1b21d1d has failed:

On branch mergify/bp/8.16/pr-14574
Your branch is up to date with 'origin/8.16'.

You are currently cherry-picking commit 1b21d1dbf.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   NOTICE.txt
	modified:   internal/beatcmd/beat_test.go
	modified:   internal/beatcmd/reloader.go
	modified:   internal/beatcmd/reloader_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   go.mod
	both modified:   go.sum

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally