Why, while developing a module, do I get an error for not specifying a handler when one exists?
Drupal 8 made the decision to be fast by default and use the APC Classloader if it is available.  It is faster, but it doesn't necessarily catch changes as you make them.  To not have to restart your web server during development, you can set your local environment to not try to use a better-for-production class loader.

For instance, in local.settings.php add:

$settings['class_loader_auto_detect'] = FALSE;


As documented in settings.php:


  If the APC extension is not detected, either because APC is missing or
  because auto-detection has been disabled, auto-loading falls back to
  Composer's ClassLoader, which is good for development as it does not
  break when code is moved in the file system.


In the issue where this change was made, Berdir warned that it would stymie developers.  More background information at what to do when developing a Drupal 8 module and a class file just isn't being loaded even though it is definitely there.