pkg/registry: export ErrNotFound

[malt3]

Custom BlobHandler implementations need access to this error. Otherwise it's impossible for them to signal missing blobs.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.05%. Comparing base (8b3c303) to head (be12354).
⚠️ Report is 55 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/registry/blobs.go	37.50%	4 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (8b3c303) and HEAD (be12354). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (8b3c303)	HEAD (be12354)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2176       +/-   ##
===========================================
- Coverage   71.67%   53.05%   -18.63%     
===========================================
  Files         123      164       +41     
  Lines        9935    10940     +1005     
===========================================
- Hits         7121     5804     -1317     
- Misses       2115     4429     +2314     
- Partials      699      707        +8     

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[malt3]

The coverage report looks invalid. I didn't add any new lines.