__win_check_git_update prints stdout, but not stderr on failure

Having an issue with `git_helper.py` disappearing and updates failing, the logs weren't very helpful.
Tracked it down to ~285 in `manager_core.py` where the stderr becomes `_` and isn't printed in the `"Fetch error:` block below.

Start in manager_core.py around line 285 and inspect the "Fetch error:" block in __win_check_git_update, then review git_helper.py for the update context. Reproduce a failed update and confirm that stderr is included in the failure logs alongside the existing error information.

Written by the indexing model from the issue text.