Error handling helpers

[mlms13]

• Adds optional error handling middleware that can be easily attached to a router
• Adds specific error types that trigger appropriate status codes (400, 401, 403, 404, 500, 501, 502, 503, 504)
• Defaults to 500 status for unhandled errors that don't specify a code
• Includes a debug mode which sends the full stack trace to the client