Provide a comprehensive message when a save-code-now request fails for know limitations

Typically a SCN request targeting a large enough git repo that the max packfile size is hit should report the reason for the failure back to the user.

We should also list and categorize all the reasons that could end with a failed SCN request and make sure a comprehensive error is reported to the SCN user.

Related to swh/infra/sysadm-environment#4390