Lack of information on error in GenericData#getSchemaName

When the type of a datum cannot be determined in GenericData#getSchemaName, an AvroRuntimeException is thrown (correctly), and the message of the exception contains the string representation of the datum.

In the odd case that the datum is something that totally doesn't belong there, and it's toString representation doesn't clearly show what it is, it's very difficult to determine what the actual type of the error datum is.

Including the class name of the unresolvable datum would make debugging situations like this much easier.