obexd: Add a detailed failure message for exchanging business cards

When sending the ExchangeBusinessCards() command, the command returns
a failure.  It isn't clear what that failure is.  Upon looking through
the code, it is obvious the function is not implemented.

This patch just adds an extra detail message 'Not Implemented' to make
the failure a little more clear about what the problem is.
diff --git a/obexd/client/opp.c b/obexd/client/opp.c
index 3c2801a..92785f6 100644
--- a/obexd/client/opp.c
+++ b/obexd/client/opp.c
@@ -117,7 +117,8 @@
 static DBusMessage *opp_exchange_business_cards(DBusConnection *connection,
 					DBusMessage *message, void *user_data)
 {
-	return g_dbus_create_error(message, ERROR_INTERFACE ".Failed", NULL);
+	return g_dbus_create_error(message, ERROR_INTERFACE ".Failed",
+							"Not Implemented");
 }
 
 static const GDBusMethodTable opp_methods[] = {