Issue #1742740 by Staratel: Fixed Add magic method __isset() to FeedsConfigurable to use default entity wrapping.

- DRUPAL-7--1-0-alpha2 http://github.com/lxbarth/feeds/tree/DRUPAL-7--1-0-alpha2
and
- DRUPAL-7--2-0-alpha1 http://github.com/lxbarth/feeds/tree/DRUPAL-7--2-0-alpha1

Add existing() method that allows for making sure that a configurable actually exists when executing a method on it.
Usage:
try {
  ->existing()->action();
} catch(FeedsNotExistingException ) {}

Fix notices from setConfig() by making sure that all default values are present.