Changed Exception wording to be more clear

framework/base/Component.php

@@ -174,7 +174,7 @@ class Component extends Object
 				}
 			}
 		}
-		throw new InvalidCallException('Unsetting read-only property: ' . get_class($this) . '.' . $name);
+		throw new InvalidCallException('Unsetting an unknown or read-only property: ' . get_class($this) . '.' . $name);
 	}
 
 	/**