Failing git commands lose stdout, so errors print empty

`Git.__call__`'s error handler prints only `e.stderr` (core.py:50), but git writes conflict/failure detail to stdout, so a failing command reports nothing useful — unlike the success path in `callgit`, which returns `r.stdout + r.stderr`.

Repro: in a repo where the merge conflicts, `Git(path).merge('upstream/main')` prints `ERROR: git merge upstream/main:` with an empty message and returns `None`; git's own `CONFLICT (content): Merge conflict in ...` / `Automatic merge failed` lines are lost.