Better error reporting for broken image builders

[efahl]

See https://forum.openwrt.org/t/owut-openwrt-upgrade-tool/200035/25

It appears that the image builder is on the correct rev, but it's internally broken. See if we can detect those cases and say something like this:

The image builder appears to be broken.
Monitor with 'owut check', noting 'Version-to SNAPSHOT r26792-646ebbd32c (kernel 6.6.35)'
When you see the "r-number" bump up beyond 26792, then try again.