The error message in object_class_property_add() was copied from
object_property_add() in commit 16bf7f52. Clarify that it is
about a class, not an object.

While here, have the format string in both functions to fit in a
single line for better grep-ability, despite the checkpatch warning.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <157287383591.234942.311840593519058490.stgit@bahia.tlslab.ibm.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>