Add a linter rule to prevent exceptions from being thrown in a toString method

Summary: Fixes T8207. It is not possible to throw an exception from within the __toString() method. Add a linter rule to prevent this from happening.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8207

Differential Revision: https://secure.phabricator.com/D12854