Make ErrSchema1 checkable via errors.Is()

[Laitr0n]

The TestGetSchema1 said it should fail based on media type while gave the unsupported MediaType. But it not return the true.

This fixes that problem by implementing an Is() function on ErrSchema1
so that errors.Is() can properly identify the error as an ErrSchema1.
Usage can now be: errors.Is(err, &ErrSchema1{schema : "foo"})

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[imjasonh]

cc @jonjohnsonjr

[codecov-commenter]

Codecov Report

Merging #1721 (b75f624) into main (037ab31) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head b75f624 differs from pull request most recent head e81effa. Consider uploading reports for the commit e81effa to get more accurate results

@@            Coverage Diff             @@
##             main    #1721      +/-   ##
==========================================
+ Coverage   71.99%   72.04%   +0.04%     
==========================================
  Files         121      121              
  Lines        9768     9764       -4     
==========================================
+ Hits         7032     7034       +2     
+ Misses       2060     2056       -4     
+ Partials      676      674       -2     

Impacted Files	Coverage Δ
pkg/v1/remote/descriptor.go	80.89% <100.00%> (-0.83%)	⬇️

... and 1 file with indirect coverage changes

[jonjohnsonjr]

I think embedding the mediaType that we saw is both unnecessary and makes this really complicated.

Can you update this to just be:

var ErrSchema1 = errors.New("unsupported schema1 mediaType, see https://github.com/google/go-containerregistry/issues/377")

And fix any tests that it breaks?

[Laitr0n]

I think embedding the mediaType that we saw is both unnecessary and makes this really complicated.

Can you update this to just be:

var ErrSchema1 = errors.New("unsupported schema1 mediaType, see https://github.com/google/go-containerregistry/issues/377")

And fix any tests that it breaks?

OK. I will done.

[jonjohnsonjr]

Oh that's clever. Added a couple suggestions so that we can keep the exact same error string but have the %w wrapping make errors.Is work.

I have a feeling someone, somewhere is checking for the string value of this error specifically because errors.Is doesn't work, so I want to avoid breaking that.

[jonjohnsonjr]

thanks!