Attempting to use projectless Git repos for a project results in unhelpful error messages

For (a contrived) example:

1. Launchpad allows me to push code to e.g. lp:~myself/+git/mystuff
2. Launchpad allows me to create a project called e.g. 'mystuff'
3. Launchpad does NOT allow me to set the git repo of that project to that code branch, but it's unclear exactly why

If I type that code url into the 'Choose' box on the project's 'Configure Code' page, I get an error: No items matched "~myself/+git/mystuff".

If I type that code url into the form anyway, and submit it, I get another unhelpful error: Invalid value

More accurate/descriptive messages would be helpful here. I understand the limitation is intentional and documented[1], but there should at least be a link to that documentation when the error occurs, ideally with a short summary/explanation directly on the page where the error occurs.

[1] https://help.launchpad.net/Code/Git