just thorw a regular exception

I'm not expecting a user to do anything actionable (other than open an issue) on this error so a regular Exception should be ok.

bin/systemlib.php

@@ -7068,7 +7068,7 @@ class RecursiveIteratorIterator implements OuterIterator, Traversable {
   public function getInnerIterator() {
     $it = $this->iterators[count($this->iterators)-1][0];
     if (!$it instanceof RecursiveDirectoryIterator) {
-      throw new NotImplementedException(
+      throw new Exception(
         "RecursiveIteratorIterator only supports RecursiveDirectoryIterator"
       );
     }

hphp/system/classes/spl/iterators/RecursiveIteratorIterator.php

@@ -67,7 +67,7 @@ class RecursiveIteratorIterator implements OuterIterator, Traversable {
   public function getInnerIterator() {
     $it = $this->iterators[count($this->iterators)-1][0];
     if (!$it instanceof RecursiveDirectoryIterator) {
-      throw new NotImplementedException(
+      throw new Exception(
         "RecursiveIteratorIterator only supports RecursiveDirectoryIterator"
       );
     }