Introduce fail-fast option

[motatoes]

Introducing a fail-fast option

Default behaviour is to continue planning even after one of the terraform plans have failed. This option if set to true will change behaviour to fail after the first terraform plan or apply has failed

[fleroux514]

As it turns out, it seems like it's already the default behavior, except for the fact that when using a mono-repo , the workflow will currently continue when a project lock is already acquired by another PR.

I think the workflow should fail in that case to be consistent.

[fleroux514]

To add-on to my last comment, now that digger combines multiple plan outputs into a single comment, the fact that the workflow is not failing when a lock can be acquired can be confusing / misleading