Throw an error when the identifier field is not available

I accidentally mapped an empty class, then I got 2 errors: "Undefined index: in ...Doctrine/ODM/CouchDB/Mapping/ClassMetadata.php", "Fatal error: Call to a member function getValue() on a non-object in .../Doctrine/ODM/CouchDB/Mapping/ClassMetadata.php".
the erroring part is `reflFields[$this->identifier]`.
The reason is, there's no id field, it's a bit confusing and it wastes new user's time.
So there should be an error message.