block-commit defaults @base-node to the deepest backing image.  When
there is none, it fails with "Base 'NULL' not found".  Improve to
"There is no backing image".

block-commit and block-stream reject a @base argument that doesn't
resolve with "Base 'BASE' not found".  Commit 6b33f3ae "qemu-img:
Improve commit invalid base message" improved this message in
qemu-img.  Improve it here, too: "Can't find '%s' in the backing
chain".

QERR_BASE_NOT_FOUND is now unused.  Drop.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113082626.2725812-4-armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>