EntityTypeManager::getHandler() exception is not always accurate

EntityTypeManager::getHandler() can throw an exception that says:

> 'The "%s" entity type did not specify a %s handler.'

However, this can be thrown if the entity type specifies the handler of the type in question, but the class isn't found.

That can happen for example if:

- there is a typo in the class name in the entity type annotation
- the handler class doesn't autoload correctly, for example, if the namespace is incorrect
- the class doesn't exist at all

In all these circumstances, the exception message is directing the developer to the wrong problem.

This is because getHandler() calls EntityType::getHandlerClass(), which in turn calls hasHandlerClass(), and that checks the class exists:

  public function hasHandlerClass($handler_type, $nested = FALSE) {
    $handlers = $this->getHandlerClasses();
    if (!isset($handlers[$handler_type]) || ($nested && !isset($handlers[$handler_type][$nested]))) {
      return FALSE;
    }
    $handler = $handlers[$handler_type];
    if ($nested) {
      $handler = $handler[$nested];
    }
    return class_exists($handler);
  }