Backport of handle FSM.Apply errors in raftApply into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #16287 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

The below text is copied from the body of the original PR.

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The signature of the raftApply function requires that the caller unwrap the first returned value (the response from FSM.Apply) to see if it's an error. This puts the burden on the caller to remember to check two different places for errors, and we've done so inconsistently.

Update raftApply to do the unwrapping for us and return any FSM.Apply error as the error value. Similar work was done in Consul in hashicorp/consul#9991. This eliminates some boilerplate and surfaces a few minor bugs in the process:

• job deregistrations of already-GC'd jobs were still emitting evals
• reconcile job summaries does not return scheduler errors
• node updates did not report errors associated with inconsistent service discovery or CSI plugin states

Note that although most of the FSM.Apply functions return only errors (which makes it tempting to remove the first return value entirely), there are few that return bool for some reason and Variables relies on the response value for proper CAS checking.