IsSupportedReference returns descriptive error

[imjasonh]

This can be useful to determine what they need to do to make a ko
publish work.

[codecov-io]

Codecov Report

Merging #233 into master will increase coverage by 0.13%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   36.64%   36.78%   +0.13%     
==========================================
  Files          33       33              
  Lines        1438     1441       +3     
==========================================
+ Hits          527      530       +3     
  Misses        822      822              
  Partials       89       89              

Impacted Files	Coverage Δ
pkg/build/limit.go	62.50% <ø> (ø)
pkg/build/recorder.go	100.00% <ø> (ø)
pkg/build/shared.go	100.00% <ø> (ø)
pkg/commands/publisher.go	0.00% <0.00%> (ø)
pkg/build/gobuild.go	58.15% <80.00%> (+0.29%)	⬆️
pkg/internal/testing/fixed.go	69.56% <100.00%> (+1.38%)	⬆️
pkg/resolve/resolve.go	68.75% <100.00%> (ø)

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[mattmoor]

I've wanted this forever, but without strict it was prohibitively noisy. Sooooo glad to see this