Get clear errors when a deployment build fails

A build that failed because of a wrong root directory or a package index it could not reach showed an error unrelated to the cause, sometimes after minutes of retries.

• Added a clear error when a procedure's root directory does not exist, listing the folders of the repository that hold a pyproject.toml.
• Improved builds that cannot reach a package index or a git source: they stop after the first attempt instead of several minutes of retries, and explain how to build with a private registry by committing uv.lock.
• Fixed build errors that blamed the Python version when the real cause was a registry refusing credentials, a missing package or a dependency that needs another Python.