Add resync/rebuild directly in monitor callback

[nkubala]

small refactor to clean the code up a bit, this code doesn't make much sense in the dev loop IMO.

the small functional effect of this change is that instead of erroring out the dev loop if we encounter an error when creating the sync item (note: this is not the same as if an actual sync error occurred), this will now log a warn to the user when skaffold tries to add the artifact to the changeSet, but the dev loop will continue. we could also log this as an error if that seems more appropriate.

[codecov]

Codecov Report

Merging #2438 into master will increase coverage by 0.09%.
The diff coverage is 77.77%.

Impacted Files	Coverage Δ
pkg/skaffold/runner/changeset.go	100% <ø> (ø)	⬆️
pkg/skaffold/runner/dev.go	64.86% <77.77%> (+0.76%)	⬆️
pkg/skaffold/schema/versions.go	74.35% <0%> (ø)	⬆️
pkg/skaffold/schema/v1beta12/upgrade.go	100% <0%> (ø)
pkg/skaffold/schema/v1beta12/config.go	100% <0%> (ø)
pkg/skaffold/build/cluster/sources/sources.go	92.15% <0%> (+1.24%)	⬆️