chore: make error of non-existed source OCI layout more readable

[qweeah]

What this PR does / why we need it:
This PR improves error message for OCI layout.

• If the provided path doesn't exist

  • Before:

  ➜  oras cp --from-oci-layout foo:bar localhost:5000/test:copied
  Error: stat foo: no such file or directory

  • After:

  ➜  oras cp --from-oci-layout foo:bar localhost:5000/test:copied
  Error: invalid path in "foo:bar": stat foo: no such file or directory

• If the provided path is a file but not a tarfile

  • Before:

  ➜  oras cp --from-oci-layout foo:bar localhost:5000/test:copied
  Error: unexpected EOF

  • After:

  ➜  oras cp --from-oci-layout foo:bar localhost:5000/test:copied
  Error: file "foo" is not a valid tarball: unexpected EOF

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 #1156

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-commenter]

Codecov Report

Merging #1176 (aafb5c0) into main (ceb3b00) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
- Coverage   80.05%   79.87%   -0.18%     
==========================================
  Files          65       65              
  Lines        3564     3572       +8     
==========================================
  Hits         2853     2853              
- Misses        495      503       +8     
  Partials      216      216              

Files	Coverage Δ
cmd/oras/internal/option/target.go	78.30% <0.00%> (-6.40%)	⬇️

[TerryHowe]

/lgtm

[shizhMSFT]

LGTM