State engine Ensure() errors are only logged, not handled

[thp-canonical]

Right now in Overlord.Loop(), the error return of the state engine's Ensure() function are ignored:

pebble/internals/overlord/overlord.go

Lines 352 to 354 in 35e2b7c

	// in case of errors engine logs them,
	// continue to the next Ensure() try for now
	o.stateEng.Ensure()

Meanwhile, snapd handles errors - at least in the pressed case - in Ensure():

https://github.com/snapcore/snapd/blob/c59a5f6e87fe59596b14614bc422e0c3a132ca25/overlord/overlord.go#L463-L473

This has been implemented in snapd for preseeding: canonical/snapd#8190

As other components of Pebble might have the same issue (retrying Ensure() forever not resulting in a change), and implementers of managers might believe that an error returned from Ensure() will be handled by the caller, it might make sense to at least allow an optional error handler to handle Ensure() errors in the overlord loop.

Found with @flotter during code review.