Improve /auth/ behavior when a provider implementation is missing

Summary: Ref T1536. This "should never happen", but can if you're developing custom providers. Improve the robustness of this interface in the presence of missing provider implementations.

Test Plan: {F47008}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6253