Change "error finalizing incremental compilation" text and emit it as a note, not a warning

[lambdageek]

As mentioned in #151181 (comment) and #151181 (comment) the current message could be improved:

1. Right now it displays as "warning: error ..." which is confusing (is it an error or a warning)
2. It doesn't give the user a clear indication of what the consequences are
3. The current build is successful. The next build might be slower

The new message is now

note: did not finalize incremental compilation session directory ...
  |
  = help: the next build will not be able to reuse work from this compilation

I started a zulip thread #t-compiler/incremental > Ergonomics of "error finalizing incremental session"