qemu-img: Improve commit invalid base message

When trying to invoke qemu-img commit with a base image file name that
is not part of the top image's backing chain, the user receives a rather
plain "Base not found" error message. This is not really helpful because
it does not explain what "not found" means, potentially leaving the user
wondering why qemu cannot find a file despite it clearly existing in the
file system.

Improve the error message by clarifying that "not found" means "not
found in the top image's backing chain".

Reported-by: Ala Hino <ahino@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20161201020508.24417-1-mreitz@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
