fix: show a better error message if gimmegit can't fetch upstream repo

[dwilding]

This PR fixes a misleading error message:

$ gimmegit -u _invalid dwilding/jubilant
Getting repo details
Cloning git@github.com:dwilding/jubilant.git
Setting upstream to git@github.com:_invalid/jubilant.git
Error: Unable to clone repo. Do you have access to the repo? Is SSH correctly configured?

The new error message is:

Error: Unable to fetch upstream repo. Do you have access to the repo? Is SSH correctly configured?