Make dedicated servers not pop up that annoying SDL error window

So they don't mess with shell scripts that expect SRB2 to exit when it crashes (like most other programs)

src/sdl/i_system.c

@@ -358,9 +358,10 @@ static void I_ReportSignal(int num, int coredumped)
 
 	I_OutputMsg("\nProcess killed by signal: %s\n\n", sigmsg);
 
-	SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
-		"Process killed by signal",
-		sigmsg, NULL);
+	if (!M_CheckParm("-dedicated"))
+		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
+			"Process killed by signal",
+			sigmsg, NULL);
 }
 
 #ifndef NEWSIGNALHANDLER
@@ -2202,9 +2203,10 @@ static void newsignalhandler_Warn(const char *pr)
 
 	I_OutputMsg("%s\n", text);
 
-	SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
-		"Startup error",
-		text, NULL);
+	if (!M_CheckParm("-dedicated"))
+		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
+			"Startup error",
+			text, NULL);
 
 	I_ShutdownConsole();
 	exit(-1);
@@ -2405,9 +2407,10 @@ void I_Error(const char *error, ...)
 			// Implement message box with SDL_ShowSimpleMessageBox,
 			// which should fail gracefully if it can't put a message box up
 			// on the target system
-			SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
-				"SRB2 "VERSIONSTRING" Recursive Error",
-				buffer, NULL);
+			if (!M_CheckParm("-dedicated"))
+				SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
+					"SRB2 "VERSIONSTRING" Recursive Error",
+					buffer, NULL);
 
 			W_Shutdown();
 			exit(-1); // recursive errors detected
@@ -2449,9 +2452,10 @@ void I_Error(const char *error, ...)
 	// Implement message box with SDL_ShowSimpleMessageBox,
 	// which should fail gracefully if it can't put a message box up
 	// on the target system
-	SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
-		"SRB2 "VERSIONSTRING" Error",
-		buffer, NULL);
+	if (!M_CheckParm("-dedicated"))
+		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
+			"SRB2 "VERSIONSTRING" Error",
+			buffer, NULL);
 	// Note that SDL_ShowSimpleMessageBox does *not* require SDL to be
 	// initialized at the time, so calling it after SDL_Quit() is
 	// perfectly okay! In addition, we do this on purpose so the