Have dependency graph throw an exception rather than return nothing when no edge nodes are present

app/Util/DependencyGraph.php

@@ -105,6 +105,11 @@ class DependencyGraph
 			}
 		}
 
+		if (sizeof($state) == 0)
+		{
+			throw new \Exception('Cannot solve dependency graph -- no edge nodes. Circular dependency?');
+		}
+
 		// Then walk through the array and insert each node's dependencies before
 		// it recursively.
 		$iter = 0;