Improve errors associated with adding new login providers

Summary:
Ref T1536.

• When users try to add a one-of provider which already exists, give them a better error (a dialog explaining what's up with reasonable choices).
• Disable such providers and label why they're disabled on the "new provider" screen.

Test Plan:
{F47012}

{F47013}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6256