[7.x](backport #25743) Don't include full ES index template in errors

[mergify]

This is an automatic backport of pull request #25743 done by Mergify.
Cherry-pick of 766e303 has failed:

On branch mergify/bp/7.x/pr-25743
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 766e30313.
  (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:   CHANGELOG.next.asciidoc
	modified:   libbeat/tests/system/test_template.py

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/template/load.go

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #26318 updated

• Start Time: 2021-06-15T17:43:04.844+0000

• Duration: 139 min 52 sec

• Commit: a202427

Test stats 🧪

Test	Results
Failed	0
Passed	47481
Skipped	5207
Total	52688

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	47481
Skipped	5207
Total	52688

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/7.x/pr-25743 upstream/mergify/bp/7.x/pr-25743
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-25743

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[jsoriano]

Hey @andrewkroh, I think this is ready to be merged 🙂