git-sync-bench: exit non-zero on failed runs, drop -1 sentinels

Two reporting bugs:

• run() returned nil after printing the report regardless of outcome, so the process exited 0 even when every benchmark run failed — a failure could pass for success in CI. Return an error (after printing the report) when any run failed.
• The aggregate Min* fields start at -1 as an "unset" sentinel updated by the first qualifying run. With no successful (or no batched) run the sentinel survived into the report, emitting minWallMillis=-1 etc. Clear the sentinels to 0 when there were no qualifying runs.