FR: `jj absorb` should explain why it could not absorb some changes

Is your feature request related to a problem? Please describe. When jj absorb can't unambiguously determine where to absorb a change, it prints

Nothing changed.

Describe the solution you'd like Instead, I think it would be helpful to print something like:

Some changes could not be absorbed because no appropriate target commits could be identified

Some changes could not be absorbed because commits tx and vu are both possible targets

This message in particular makes it much easier to run jj absorb --into vu after determining which of the ambiguous changes should be the target

Also, it would be even better if there were a way to be more precise about what changes could not be absorbed than "some changes", but I'm not sure how feasible that is.