Closes #126

This uses promise.allSettled() instead of promise.all() when dispatching the same message to multiple handlers on the same instance. This is done as there is no coupling between handlers and failure in one shouldn't affect processing in another.

Note that this still throws all errors from the dispatch that are now inside the HandlerDispatchRejected error in the reasons attribute.

The message will still be retried, so it's still important as always to ensure message handling is idempotent.