Error message for uploading to protected branch

Branches can be protected from commits being directly pushed to them.
When trying to do so anyway, the gin-cli yields a misleading error message:
```
upload failed: changes were made on the server that have not been downloaded; run 'gin download' to update local copies
```
In this case following the instructions and running `gin download` does not resolve the issue of not being able to push commits.
This is linked to #52