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.phabricator.com/D7151