change error value "fatal io error" to just "io error"

"fatal" was meant as "we need fatal for the connection, it will be dropped".
but it sounds more serious, as if something needs to be fixed.

hopefully enough for issue #39 by ArnoSen

smtpserver/server.go

@@ -63,7 +63,7 @@ var xlog = mlog.New("smtpserver")
 
 // We use panic and recover for error handling while executing commands.
 // These errors signal the connection must be closed.
-var errIO = errors.New("fatal io error")
+var errIO = errors.New("io error")
 
 // If set, regular delivery/submit is sidestepped, email is accepted and
 // delivered to the account named mox.