Enumerate some errcode-adjacent function results

Enumerate some errcode-adjacent function results

To reduce review friction; clarify when result codes matter, force
callers to worry about them, and prevent them from getting jumbled with
other error code types.

(In particular, the cache code was returning `EBUSY` to signal rsync
deferral. Since the code frequently propagates errno, it risked regular
standard library `EBUSY`s being mistaken by rsync deferrals.)

Also, remove negative error codes when they're not needed. (Though
error codes themselves are steadily becoming slop.)