`DefaultErrorHandler` renamed to `FallbackErrorHandler`

DefaultErrorHandler has been renamed to FallbackErrorHandler to better reflect its role as the handler of last resort when no specific error handling is performed.

A deprecated type alias is provided for one release to ease migration. To update your code:

// 0.18
world.insert_resource(DefaultErrorHandler(my_error_handler));

// 0.19
world.insert_resource(FallbackErrorHandler(my_error_handler));

The default_error_handler method has similarly been renamed to fallback_error_handler.