Add tests for httpResponseToError
Add tests for httpResponseToError

It's high time we started collecting error samples and
documenting what happens.

NOTE: This is NOT an API commitment, we can change the error
types and messages any time.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image