Error instead of opening interactive prompt with --force init

[TadCordle]

Fixes #3221.

skaffold init --force now shows an error if all builder/image pairs can't be automatically resolved.

[codecov]

Codecov Report

Merging #3252 into master will increase coverage by 0.26%.
The diff coverage is 75%.

Impacted Files	Coverage Δ
pkg/skaffold/initializer/init.go	58.44% <75%> (+0.27%)	⬆️
pkg/skaffold/build/gcb/spec.go	93.75% <0%> (-6.25%)	⬇️
cmd/skaffold/app/cmd/run.go	47.05% <0%> (-5.58%)	⬇️
pkg/skaffold/build/custom/custom.go	58.13% <0%> (-4.37%)	⬇️
pkg/skaffold/docker/image.go	76.36% <0%> (ø)	⬆️
cmd/skaffold/app/cmd/flags.go	100% <0%> (ø)	⬆️
pkg/skaffold/docker/containers.go	67.44% <0%> (ø)	⬆️
pkg/skaffold/build/cluster/sources/gcs.go	0% <0%> (ø)	⬆️
pkg/skaffold/gcp/client.go	0% <0%> (ø)	⬆️
pkg/skaffold/build/gcb/buildpacks.go	100% <0%> (ø)
... and 2 more

[dgageot]

LGTM with a nit