Merge branch 'ds/midx-write-fixes' into jch

Fixes multiple crashes around midx write-out codepaths.

Comments?

* ds/midx-write-fixes:
  midx-write: simplify error cases
  midx-write: reenable signed comparison errors
  midx-write: use uint32_t for preferred_pack_idx
  midx-write: use cleanup when incremental midx fails
  midx-write: put failing response value back
  midx-write: only load initialized packs

Author: gitster