Error message for uploading to protected branch

[JuliaSprenger]

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