Error interface adjustments to prepare for next major version

[Firehed]

This moves an endpoint's handleException method into its own interface, so that endpoints explicitly indicate that they are capable of handling their own errors. Most endpoints will not implement this interface.

The main EndpointInterface has been adjusted to extend this new interface, so that existing behavior will remain unchanged. In the next major version, that will be removed, resulting in two main changes:

• Implementing a new v4 endpoint will require less work, less code, and less boilerplate
• Existing v3 endpoints that have NOT been updated to implement that interface when migrating to v4 will no longer see their error handlers called

Note that the only migration step that will be required in v4 as a result of this change is adding HandlesOwnErrorsInterface to the endpoint's existing implements list.