fix: empty error on bump failure

before:

```
2nd git.commit error: ""
```

after

```
2nd git.commit error: "On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean
"
```

commitizen/commands/bump.py

@@ -329,7 +329,8 @@ def __call__(self):  # noqa: C901
             cmd.run(git_add_changelog_and_version_files_command)
             c = git.commit(message, args=self._get_commit_args())
         if c.return_code != 0:
-            raise BumpCommitFailedError(f'2nd git.commit error: "{c.err.strip()}"')
+            err = c.err.strip() or c.out
+            raise BumpCommitFailedError(f'2nd git.commit error: "{err}"')
 
         if c.out:
             if self.git_output_to_stderr: