code: public function resolve($handler)
  	{
  		if (!is_string($handler)) {
  			return $handler;
  		}
  
  		$handler = $this->container->resolve($handler);
  
  		if (!is_callable($handler) && !$handler instanceof ErrorHandlerInterface) {
  			$type = is_object($handler) ? get_class($handler) : gettype($handler);
  			throw new \UnexpectedValueException("Resolved error handler is not a valid handler - must be callable or an instance of Autarky\Errors\ErrorHandlerInterface, $type given");
  		}
  
  		return $handler;
  	}
docs: Resolve a handler.
  
  @param  mixed $handler
  
  @return ErrorHandlerInterface|callable