fix: update error message for repo ls

[sajayantony]

What this PR does / why we need it:
Improves the error message to show the registry and namespace when catalog listing fails.

$ oras repo ls docker.io
Error: could not list repositories for "registry-1.docker.io"
GET "https://registry-1.docker.io/v2/_catalog": GET "https://auth.docker.io/token?scope=registry%3Acatalog%3A%2A&service=registry.docker.io": response status code 400: Bad Request

$ oras repo ls docker.io/python
Error: could not list repositories for "registry-1.docker.io" with prefix "python/"
GET "https://registry-1.docker.io/v2/_catalog": GET "https://auth.docker.io/token?scope=registry%3Acatalog%3A%2A&service=registry.docker.io": response status code 400: Bad Request

[codecov-commenter]

Codecov Report

Merging #1169 (48ce099) into main (94e32e8) will decrease coverage by 0.04%.
The diff coverage is 70.00%.

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@@            Coverage Diff             @@
##             main    #1169      +/-   ##
==========================================
- Coverage   80.03%   80.00%   -0.04%     
==========================================
  Files          65       65              
  Lines        3551     3560       +9     
==========================================
+ Hits         2842     2848       +6     
- Misses        494      495       +1     
- Partials      215      217       +2     

Files	Coverage Δ
cmd/oras/root/repo/ls.go	88.23% <70.00%> (-4.63%)	⬇️

[TerryHowe]

/lgtm