Running `opensafely launch` should handle not being in the right directory better

[bloodearnest]

A user ran opensafely launch in their windows home directory.

This didn't work, as it tried to mount some stuff it didn't have permissions for, and gave a confusing error message.

For some commands, you need to be in an opensafely project directory to work. e.g. opensafely run. The opensafely run code checks for this explicitly, and errors.

But the same is also true for other opensafely commands, probably: launch, clean, codelists.

We should detect if we are in an opensafely project dir by looking for a project.yaml, and error with a more helpful error message in these cases. This should avoid further user confusion.

Came in via tech support in this Slack thread.