Rollup merge of #107746 - Nathan-Fenner:nathanf/clean-up-adjust_fulfillment_errors...

Split fn_ctxt/adjust_fulfillment_errors from fn_ctxt/checks

This is a follow-up from #106477, addressing a small number of the `FIXME`s that were added, by moving some functions into the new(er) `adjust_fulfillment_errors` module.

More cleanup is possible for this file (and I'll hopefully get around to doing some of that soon) but the very first thing is to just move these functions out.

There should be no "real" changes in this PR, besides minor adjustments to imports and the functions being transferred.