chore(ux): show registry error and hint for dockehub

[qweeah]

What this PR does / why we need it:
This PR

1. adds prefix to errors returned by registry. E.g. invalid creds for fetching a manifest

$ oras manifest fetch-config localhost:7000/repo:tag -u hello -p 123 >>
WARNING! Using --password via the CLI is insecure. Use --password-stdin.
Error response from registry: unauthorized: authentication required

2. gives recommendation if namespace is missing in a Dockerhub reference.

$ oras pull docker.io/apline:latest
Error response from registry: failed to resolve latest: unauthorized: authentication required: [map[Action:pull Class: Name:apline Type:repository]]
Namespace is missing, do you mean `oras pull docker.io/library/apline:latest`?

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #1216

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (04f0c33) 80.65% compared to head (e602b89) 81.03%.

Files	Patch %	Lines
cmd/oras/internal/errors/errors.go	90.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1222      +/-   ##
==========================================
+ Coverage   80.65%   81.03%   +0.37%     
==========================================
  Files          67       67              
  Lines        3644     3727      +83     
==========================================
+ Hits         2939     3020      +81     
- Misses        493      496       +3     
+ Partials      212      211       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[shizhMSFT]

LGTM with suggestions