Tweak parser warning

Marlin/src/core/serial.h

 #define SERIAL_ERROR_START()        serial_error_start()
 #define SERIAL_EOL()                SERIAL_CHAR('\n')
 
-#define SERIAL_ECHO_MSG(S)          do{ SERIAL_ECHO_START(); SERIAL_ECHOLNPGM(S); }while(0)
-#define SERIAL_ERROR_MSG(S)         do{ SERIAL_ERROR_START(); SERIAL_ECHOLNPGM(S); }while(0)
+#define SERIAL_ECHO_MSG(V...)       do{ SERIAL_ECHO_START(); SERIAL_ECHOLNPAIR(V); }while(0)
+#define SERIAL_ERROR_MSG(V...)      do{ SERIAL_ERROR_START(); SERIAL_ECHOLNPAIR(V); }while(0)
 
 #define SERIAL_ECHO_SP(C)           serial_spaces(C)
 
 void serial_spaces(uint8_t count);
 
 void print_bin(const uint16_t val);
-
 void print_xyz(const float &x, const float &y, const float &z, PGM_P const prefix=nullptr, PGM_P const suffix=nullptr);
 
 inline void print_xyz(const xyz_pos_t &xyz, PGM_P const prefix=nullptr, PGM_P const suffix=nullptr) {

Marlin/src/gcode/gcode.cpp

         case 800: parser.debug(); break;                          // G800: GCode Parser Test for G
       #endif
 
-      default: parser.unknown_command_error(); break;
+      default: parser.unknown_command_warning(); break;
     }
     break;
 
         case 7219: M7219(); break;                                // M7219: Set LEDs, columns, and rows
       #endif
 
-      default: parser.unknown_command_error(); break;
+      default: parser.unknown_command_warning(); break;
     }
     break;
 
       #if ENABLED(WIFI_CUSTOM_COMMAND)
         if (wifi_custom_command(parser.command_ptr)) break;
       #endif
-      parser.unknown_command_error();
+      parser.unknown_command_warning();
   }
 
   if (!no_ok) queue.ok_to_send();

Marlin/src/gcode/parser.cpp

 
 #endif // CNC_COORDINATE_SYSTEMS
 
-void GCodeParser::unknown_command_error() {
-  SERIAL_ECHO_START();
-  SERIAL_ECHOLNPAIR(MSG_UNKNOWN_COMMAND, command_ptr, "\"");
+void GCodeParser::unknown_command_warning() {
+  SERIAL_ECHO_MSG(MSG_UNKNOWN_COMMAND, command_ptr, "\"");
 }
 
 #if ENABLED(DEBUG_GCODE_PARSER)
     #else
       SERIAL_ECHOPAIR(" args: { ", command_args, " }");
     #endif
-    if (string_arg) SERIAL_ECHOPAIR(" string: \"", string_arg, "\"");
+    if (string_arg) {
+      SERIAL_ECHOPAIR(" string: \"", string_arg);
+      SERIAL_CHAR('"');
+    }
     SERIAL_ECHOLNPGM("\n");
     for (char c = 'A'; c <= 'Z'; ++c) {
       if (seen(c)) {

Marlin/src/gcode/parser.h

 
   static inline feedRate_t value_feedrate() { return MMM_TO_MMS(value_linear_units()); }
 
-  void unknown_command_error();
+  void unknown_command_warning();
 
   // Provide simple value accessors with default option
   static inline float    floatval(const char c, const float dval=0.0)   { return seenval(c) ? value_float()        : dval; }