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
https://github.com/hashicorp/consul/pull/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.
