Re-organized status messages to be more coherent and centralized.

- Reorganized all of the status message feedback from both the g-code
parser and settings modules to be centralized into two message modules:
status feedback from executing a line and warnings for misc feedback.

- Pulled out the printPgmString() messages in settings.c and placed it
into the new module. (settings_dump() not moved).

- Some other minor edits. Renaming defines, comment updates, etc.

protocol.h

@@ -21,17 +21,28 @@
 #ifndef protocol_h
 #define protocol_h
 
+
+#define LINE_BUFFER_SIZE 50
+
+// Define Grbl status codes.
 #define STATUS_OK 0
+// Critical error codes. Greater than zero.
 #define STATUS_BAD_NUMBER_FORMAT 1
 #define STATUS_EXPECTED_COMMAND_LETTER 2
 #define STATUS_UNSUPPORTED_STATEMENT 3
 #define STATUS_FLOATING_POINT_ERROR 4
 #define STATUS_MODAL_GROUP_VIOLATION 5
-#define STATUS_INVALID_COMMAND 6
+#define STATUS_INVALID_STATEMENT 6
 #define STATUS_SETTING_DISABLED 7
 #define STATUS_HARD_LIMIT 8
+// Non-critical error codes. Less than zero.
+#define STATUS_SETTING_INVALID -1
+#define STATUS_SETTING_STEPS_NEG -2
+#define STATUS_SETTING_STEP_PULSE_MIN -3
 
-#define LINE_BUFFER_SIZE 50
+// Define Grbl warning message codes
+#define WARNING_HOMING_ENABLE 1
+#define WARNING_SETTING_READ_FAIL 2
 
 // Initialize the serial protocol
 void protocol_init();
@@ -41,12 +52,12 @@ void protocol_init();
 void protocol_process();
 
 // Executes one line of input according to protocol
-uint8_t protocol_execute_line(char *line);
+int8_t protocol_execute_line(char *line);
 
 // Checks and executes a runtime command at various stop points in main program
 void protocol_execute_runtime();
 
-// Prints g-code parser status message.
-void protocol_status_message(int8_t status_code);
+// Prints any warning messages.
+void protocol_warning_message(int8_t warning_code);
 
 #endif