Improve the logs when there's no skaffold.yaml

[dgageot]

Before:

$ skaffold build
WARN[0000] Your Skaffold version might be too old. Download the latest version (0.33.0) at https://storage.googleapis.com/skaffold/releases/latest/skaffold-darwin-amd64
Complete in 394.890149ms
FATA[0000] creating runner: parsing skaffold config: read skaffold config: open skaffold.yaml: no such file or directory

After:

$ skaffold build
FATA[0000] creating runner: [skaffold.yaml] not found. You might need to run `skaffold init`

[codecov]

Codecov Report

Merging #2467 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/build.go	88.46% <ø> (-1.54%)	⬇️
cmd/skaffold/app/cmd/runner.go	69.23% <100%> (+0.8%)	⬆️