Improve error handling in UNIX socket server

Log errors, and drop unnecessary replies when shutting down a
connection.

server/unixserver.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -159,11 +160,11 @@ func (s *SocketServer) handle(nc net.Conn) {
 			return
 		case nargs == 2 && cmd == "auth":
 			if err := s.handleAuth(c, parts[1]); err != nil {
-				log.Printf("error from %s: %v", nc.RemoteAddr(), err)
+				log.Printf("error in auth: %v", err)
 				return
 			}
 		default:
-			c.PrintfLine("syntax error from %s", nc.RemoteAddr())
+			log.Printf("syntax error")
 			return
 		}
 	}
@@ -172,8 +173,7 @@ func (s *SocketServer) handle(nc net.Conn) {
 func (s *SocketServer) handleAuth(c *textproto.Conn, line []byte) error {
 	var req auth.Request
 	if err := s.codec.Decode(line, &req); err != nil {
-		c.PrintfLine("syntax error")
-		return errors.New("syntax error")
+		return fmt.Errorf("decoding error: %v", err)
 	}
 
 	// Set a timeout for the request.