Make helpful error message when user tries to join an invite-only project

[tiwane]

If you try to join an invite-only project, you see a 422 error, which is confusing for users. If someone tries to join an invite-only project (eg https://www.inaturalist.org/projects/tony-s-test/), they should see an error message that tells them it's not possible for them to join, eg

You must be invited to this project in order to join it.