PropertyModifierJsonDecode: fail on error...

...and give some details

fixes #13733

library/Director/PropertyModifier/PropertyModifierJsonDecode.php

@@ -36,7 +36,7 @@ class PropertyModifierJsonDecode extends PropertyModifierHook
         }
 
         $decoded = @json_decode($value);
-        if ($decoded === null && JSON_ERROR_NONE === json_last_error()) {
+        if ($decoded === null && JSON_ERROR_NONE !== json_last_error()) {
             switch ($this->getSetting('on_failure')) {
                 case 'null':
                     return null;
@@ -45,7 +45,8 @@ class PropertyModifierJsonDecode extends PropertyModifierHook
                 case 'fail':
                 default:
                     throw new InvalidPropertyException(
-                        'JSON decoding failed for %s',
+                        'JSON decoding failed with "%s" for %s',
+                        $this->getLastJsonError(),
                         $value
                     );
             }
@@ -53,4 +54,23 @@ class PropertyModifierJsonDecode extends PropertyModifierHook
 
         return $decoded;
     }
+
+    // TODO: just return json_last_error_msg() for PHP >= 5.5.0
+    protected function getLastJsonError()
+    {
+        switch (json_last_error()) {
+            case JSON_ERROR_DEPTH:
+                return 'The maximum stack depth has been exceeded';
+            case JSON_ERROR_CTRL_CHAR:
+                return 'Control character error, possibly incorrectly encoded';
+            case JSON_ERROR_STATE_MISMATCH:
+                return 'Invalid or malformed JSON';
+            case JSON_ERROR_SYNTAX:
+                return 'Syntax error';
+            case JSON_ERROR_UTF8:
+                return 'Malformed UTF-8 characters, possibly incorrectly encoded';
+            default:
+                return 'An error occured when parsing a JSON string';
+        }
+    }
 }