Raise an error when the 'for' tag cannot iterate over a value.

This makes debugging a bit easier, because otherwise there would only be
a PHP error about "Invalid argument supplied for foreach" without giving
any clues which value it could possibly be.

h2o/tags.php

@@ -130,6 +130,12 @@ function __construct($argstring, $parser, $position) {
     function render($context, $stream) {
         $iteratable = $context->resolve($this->iteratable);
 
+        // Make debugging a bit easier
+        if( ! is_array( $iteratable ) && ! $iteratable instanceof Traversable ) {
+            $repr = (is_object( $iteratable ) ? '<class: ' . get_class( $iteratable ) . '>' : (string)$iteratable );
+            throw new InvalidArgumentException("The for tag cannot iterate over the value: " . $repr);
+        }
+
         if ($this->reversed)
             $iteratable = array_reverse($iteratable);