Improve error message for non-renku projects

[Panaetius]

see https://renku.discourse.group/t/the-target-project-is-not-a-renku-project/988 for more details.

but the error when trying to use renku on a project that doesn't have .renku or .renku/metadata should mention using renku init to turn the project into a renku project.

Might also need some change on the UI if the error happens in the core service.