Fix an issue in legalpad when signing with a registered email address

Summary: If you're logged out and try to sign a document with a registered email address, we don't handle telling you to login correctly.

Test Plan: Tried to sign with a registered address, got a helpful dialog instead of a fatal.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12544