Log estimated pack size and full error on every push attempt

Two new --verbose log lines so the sequence of bootstrap pushes is visible end-to-end and we can tell whether a server error is a disguised body-size issue:

• "bootstrap batch push attempting" before each PushPack call, carrying the header-derived size estimate (objects × 750 bytes) and the configured target limit. Lets you see the trajectory of pack sizes across subdivisions: 1.6 GB → 800 MB → 400 MB ...

• "bootstrap batch push failed" on every failure, with the same estimate plus a will_subdivide flag and the full err.Error() (which already includes status code, Cf-Ray, and response body via gitproto.httpError). Previously we only saw the propagated final error; intermediate failures were silent unless they happened to match the body-limit heuristic.

checkPackSizeAndSubdivide now also returns the estimate so the caller can attach it to the "attempting" log even when no subdivision was triggered.