R1:5799e8e2de67

Provide better strings in policy errors and exceptions

Summary:
Ref T603. This could probably use a little more polish, but improve the quality of policy error messages.

- Provide as much detail as possible.
- Fix all the strings for i18n.
- Explain special rules to the user.
- Allow indirect policy filters to raise policy exceptions instead of 404s.

Test Plan: See screenshots.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.