When we can't store signatures, point the user at the destination.
When we can't store signatures, point the user at the destination.

Currently the error message is:
> Can not copy signatures: X-Registry-Supports-Signatures extension not supported, and lookaside is not configured

which does not tell the user whether to check the source or the destination.

Mention the destination explicitly.

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

containers/image,containers/image