Patch release v0.9.1

Hot-fix error message on gated repositories (#1015).

Context: https://huggingface.co/CompVis/stable-diffusion-v1-4 has been widely shared in the last days but since it's a gated-repo, lots of users are getting confused by the Authentification error received. Error message is now more detailed.

Full Changelog: v0.9.0...v0.9.1