Improve code guidelines for exception handler

Summary:
The class had still the autogenerated style of Laravel
instead to use our code base guidelines:

Quick cleanup:

• Drop a method only calling parent method
• Sort by alphabetical and logic order use clauses
• Add PHP 7 return types
• Document more richly arrays as type[]
• Respect Nasqueron code style:
  • Comments are 3rd person
  • Brackets on the same line

Test Plan: Run tests

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D783