Introduce severity levels for alert messages

Use the internal lcd_status_message_level for multiple severity levels
of alert messages.

This is needed to distinguish between non-critical alerts (such as FAN
ERROR) from critical ones (any heater issue). During a failure
scenario, a critical error MUST NOT be overridden by a lower-level one.

As such LCD_STATUS_CRITICAL is currently used for all heater-related
errors that result in a safety full-stop.

Firmware/ultralcd.cpp

@@ -8813,18 +8813,22 @@ void lcd_updatestatus(const char *message){
 	lcd_draw_update = 1;
 }
 
-void lcd_setalertstatuspgm(const char* message)
+void lcd_setalertstatuspgm(const char* message, uint8_t severity)
 {
-  lcd_setstatuspgm(message);
-  lcd_status_message_level = 1;
-  lcd_return_to_status();
+  if (severity > lcd_status_message_level) {
+      lcd_setstatuspgm(message);
+      lcd_status_message_level = severity;
+      lcd_return_to_status();
+  }
 }
 
-void lcd_setalertstatus(const char* message)
+void lcd_setalertstatus(const char* message, uint8_t severity)
 {
-  lcd_setstatus(message);
-  lcd_status_message_level = 1;
-  lcd_return_to_status();
+  if (severity > lcd_status_message_level) {
+      lcd_setstatus(message);
+      lcd_status_message_level = severity;
+      lcd_return_to_status();
+  }
 }
 
 void lcd_reset_alert_level()

Firmware/ultralcd.h

@@ -12,13 +12,20 @@ extern void menu_lcd_lcdupdate_func(void);
 void ultralcd_init();
 void lcd_setstatus(const char* message);
 void lcd_setstatuspgm(const char* message);
+
+//! LCD status severities
+#define LCD_STATUS_CRITICAL 2 //< Heater failure
+#define LCD_STATUS_ALERT    1 //< Other hardware issue
+#define LCD_STATUS_NONE     0 //< No alert message set
+
 //! return to the main status screen and display the alert message
 //! Beware - it has sideeffects:
 //! - always returns the display to the main status screen
 //! - always makes lcd_reset (which is slow and causes flicker)
-//! - does not update the message if there is already one (i.e. lcd_status_message_level > 0)
-void lcd_setalertstatus(const char* message);
-void lcd_setalertstatuspgm(const char* message);
+//! - does not update the message if there is one with the same (or higher) severity present
+void lcd_setalertstatus(const char* message, uint8_t severity = LCD_STATUS_ALERT);
+void lcd_setalertstatuspgm(const char* message, uint8_t severity = LCD_STATUS_ALERT);
+
 //! only update the alert message on the main status screen
 //! has no sideeffects, may be called multiple times
 void lcd_updatestatus(const char *message);