infra/go/internal/git: better error messages

The err text is always "exit status %d" which is really not useful.
So by default, this error is now annotated with the stderr output.

TEST=run_test.sh
BUG=None

Change-Id: I4f4dbb958b50a46310f0e8a852db8ae6c2f05fa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/1761349
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
diff --git a/internal/git/git.go b/internal/git/git.go
index b57561e..9793f00 100644
--- a/internal/git/git.go
+++ b/internal/git/git.go
@@ -37,7 +37,7 @@
 	var stdoutBuf, stderrBuf bytes.Buffer
 	err := CommandRunnerImpl.RunCommand(ctx, &stdoutBuf, &stderrBuf, gitRepo, "git", cmd...)
 	cmdOutput := CommandOutput{stdoutBuf.String(), stderrBuf.String()}
-	return cmdOutput, err
+	return cmdOutput, errors.Annotate(err, cmdOutput.Stderr).Err()
 }
 
 // RunGitIgnore the specified git command in the specified repo and returns