This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.
To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.
In some cases this is needed to keep the UX clean and to make sure any remote exit codes are passed through to the local process.
The most obvious example for this is when using the "remote" backend. This backend runs Terraform remotely and stream the output back to the local terminal.
When an error occurs during the remote execution, all the needed error information will already be in the streamed output. So if we then return an error ourselves, users will get the same errors twice.
By allowing the backend to specify the correct exit code, the UX remains the same while preserving the correct exit codes.
Sander van Harmelen