Return user friendly error when package doesn't exist

[rcmadhankumar]

What this PR does / why we need it:

Return user friendly error when package doesn't exist

$ kubectl get package foo.1.0.0
Error from server (NotFound): package.data.packaging.carvel.dev "foo.1.0.0" not found

$kctrl package available get -p foo/1.0.0
kctrl: Error: package.data.packaging.carvel.dev "foo.1.0.0" not found

Which issue(s) this PR fixes:

Fixes #594

Does this PR introduce a user-facing change?

NONE

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

NONE

[rcmadhankumar]

@100mik @praveenrewar can you review this?

[praveenrewar]

LGTM!

[joaopapereira]

Made some comments with some suggested changes for the PR. In general looks good.
Since we are starting to use testify in the tests, it would be good if we did update, at least the test file to use require instead of the t.Fatalf.

[joaopapereira]

LGTM

[praveenrewar]

LGTM!