Report authentication errors when pulling from SSH remote

If I try to pull from an SSH remote with `##class(SourceControl.Git.API).Pull()` and my SSH key is not accepted, there are no errors reported. The return status is ok and the output looks like:
```
Pulling from branch: main
Fetch done
Files that will be modified by git pull:
    None
```
We should check for errors in the git fetch and output them to the user.