Improve the error message for fork failures

Problem

Originally reported: https://gitlab.com/gitlab-com/dev-sub-department/section-dev-request-for-help/-/issues/8

POST /api/v4/projects/<id>/fork returns 409 Conflict response with error message: Unable to save project. Error: Invalid request

The error message is very obscure and needs to be improved.

Root cause

The root cause was a misconfiguration of gitlab_rails['object_store']. "uploads" => { enabled: false } was missing and we used the default value true for that. I suspect that we tried to send an object to object store, but the store was returning an error.

Proposal

1. Improve error message by adding extra details (like exception class, possibly the backtrace)
2. Try to reproduce the misconfiguration error and find a better way to report this problem.