[MER-877] cli: improve error message if stack branch fails (#41)

not totally sure what i'm doing but i think it works??

<img width="989" alt="Screen Shot 2022-07-06 at 1 03 48 PM" src="https://user-images.githubusercontent.com/13113118/177633714-9189dd40-0078-4d47-93d9-ef1e2039299f.png">

cmd/av/stack_branch.go

@@ -29,11 +29,6 @@ var stackBranchCmd = &cobra.Command{
 			return err
 		}
 
-		// Validate preconditions
-		if _, err := repo.RevParse(&git.RevParse{Rev: name}); err == nil {
-			return errors.Errorf("branch %q already exists", name)
-		}
-
 		// Determine important contextual information from Git
 		defaultBranch, err := repo.DefaultBranch()
 		if err != nil {
@@ -76,11 +71,7 @@ var stackBranchCmd = &cobra.Command{
 			Name:      name,
 			NewBranch: true,
 		}); err != nil {
-			logrus.WithError(err).Debugf("failed to checkout branch %q", name)
-			return errors.Errorf(
-				"failed to create branch %q (does it already exist?)",
-				name,
-			)
+			return errors.WrapIff(err, "checkout error")
 		}
 
 		branchMeta := meta.Branch{Name: name, Parent: parentBranch}

internal/git/git.go

@@ -166,9 +166,18 @@ func (r *Repo) CheckoutBranch(opts *CheckoutBranch) (string, error) {
 		args = append(args, "-b")
 	}
 	args = append(args, opts.Name)
-	if _, err := r.Git(args...); err != nil {
-		return "", err
+	res, err := r.Run(&RunOpts{
+		Args: args,
+	})
+	if err != nil {
+	    return "", err
 	}
+	if res.ExitCode != 0 {
+		logrus.WithFields(logrus.Fields{
+		    "stdout": string(res.Stdout),
+		    "stderr": string(res.Stderr),
+		}).Debug("git checkout failed")
+        return "", errors.Errorf("failed to checkout branch %q: %s", opts.Name, string(res.Stderr))	}
 	return previousBranchName, nil
 }