zm,zb: Correctly convert zbus::Error::Method to DBusError impls
This brings back functionality dropped in 986de174, where started to convert zbus::Error to always convert to the fallthrough zbus_error
variant. This would mean that proxies using fdo::Error (which is a DBusError) will not actually be able to make use of this error. The same would go for other DBusError impls as well. This is of course not nice.
So this commit fixes that by converting to the appropriate variant, if any.