Fix Pull #62 error: ClassMetadataInfo::checkUp() - Embedded documents shouldn't have an id.

The `ClassMetadataInfo:checkUp()` method added in Pull #62 checked for the presence of an identifier but didn't exempt embedded documents.

The comment of this method is now more specific and ready to be extended with the details of additional future checks.

The exception message indicates that _identifier_ refers to the _@Id_ field since this wasn't immediately apparent, and the name of the class producing the error is also included.