java_model: generate OFErrorMsg accessor for ErrorCause data

Reviewer: @Sovietaced @vishnu-emmadi @rlane
in the loxigen model, the virtual of_error_msg superclass does not have
a data field) (but all its concrete subclasses have one). This little
patch makes the java model generate a "data" contract in the interface
anyway, which is useful for error handling schemes that depend on the
type of the causing message.
why not virtual_members.append for consistency?