invalid --distribution-spec flag returns wrong error information

[qweeah]

What happened in your environment?

If an invalid distribution flag is specified by --distribution-spec, the returned error message will imply wrong image specification.

$ oras attach --artifact-type oras.test localhost:5000/command/images:foobar --distribution-spec "???"
Error: unknown image specification flag: "???"

What did you expect to happen?

The returned error message should imply wrong distribution specification.

$ oras attach --artifact-type oras.test localhost:5000/command/images:foobar --distribution-spec "???"
Error: unknown distribution specification flag: "???"

How can we reproduce it?

Use an invalid distribution flag on a command that supports tuning distribution specification.

What is the version of your ORAS CLI?

1.0.0

What is your OS environment?

20.04

Are you willing to submit PRs to fix it?

• Yes, I am willing to fix it.