Add failure reason as output to terraform plan

[LRost]

Suggestion

In the github action of terraform-apply there is the output 'failure-reason' defined, but this output is missing on terraform-plan.

In my opinion, this is also welcome to summarize why the plan stage might fail.

[dflook]

Hello @LRost, what failure reasons would be useful to you? None of the failure-reasons for the apply action can happen with the plan action.

[LRost]

Hi @dflook,

Does that need to be pre-defined or can stderr also be used?