fix: Improve error message for actions expanded with count/for_each in action_trigger

[devhindo]

fixes: #37927

This PR improves the error message displayed when an action referenced in an action_trigger block cannot be resolved to a specific instance.

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

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CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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[mildwonkey]

Thank you for working on this @devhindo !

I only want to change the error message to this if we have reason to think that is the problem - for example, if the trigger references an action when it should have been an action instance. But this will also get returned if for e.g. the action isn't in the config at all, or there's a typo on the action name. If we can't make a reasonable guess, we should return the original (generic) error.