varlink: if varlink_call() is called with ret_error_id=NULL propagate error via retur...

varlink: if varlink_call() is called with ret_error_id=NULL propagate error via return value

It's OK if callers don't want to know the varlink error string. But in
that case return the fact the call failed via the return value, as a
negative errno as usual, to make sure it's not accidentally ignored.

Similar for varlink_observe()