varlink: didn't generate a varlink error reply if a failed method call handler alread...

varlink: didn't generate a varlink error reply if a failed method call handler already did

It might happen that a method call handler already generated an error
reply and then still propagated the error back to the varlink logic.
Let's not try to generate a 2nd reply from that error code then, but
simply proceed without. This simplifies handling of errors in method
call handlers, because they can uniformly return errno-style error
codes, and only if they want return a full Varlink errror.