[Custom] Clearer message when image was not built

[dgageot]

When the custom script failed to build the image with the provided tag, it used to give an error like:

building [img]: Error parsing reference: "" is not a valid repository/tag: invalid reference format 

It's now:

building [img]: build artifact: The custom script didn't produce an image with tag [img:tag]

Will help debugs issue like #3077

Signed-off-by: David Gageot [email protected]

[codecov]

Codecov Report

Merging #3085 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

Impacted Files	Coverage Δ
pkg/skaffold/build/local/custom.go	13.33% <0%> (-6.67%)	⬇️