(opm render): refine error message to indicate expected input

[anik120]

Description of the change:

Motivation for the change:

Reviewer Checklist

• [ ] Implementation matches the proposed design, or proposal is updated to match implementation
• [ ] Sufficient unit test coverage
• [ ] Sufficient end-to-end test coverage
• [ ] Docs updated or added to /docs
• [ ] Commit messages sensible and descriptive

[kevinrizza]

@anik120 Does it make sense to be slightly more generic with this error message though? The labels drive this today, but do we not have code to look at an image and try to find the fbc, database, or manifest directory of a registry+v1 bundle image? And if not, does it make sense for the error message to make room for that improvement in the future?

[codecov[bot]]

Codecov Report

Merging #939 (13372cc) into master (9999f79) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #939   +/-   ##
=======================================
  Coverage   52.21%   52.21%           
=======================================
  Files         103      103           
  Lines        9117     9117           
=======================================
  Hits         4760     4760           
  Misses       3451     3451           
  Partials      906      906           

Impacted Files	Coverage Δ
alpha/action/render.go	64.14% <0.00%> (ø)

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[perdasilva]

closing PR as stale. Please re-open if it's still important.