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

@@ -358,6 +358,7 @@ static void I_ReportSignal(int num, int coredumped)
 
 	I_OutputMsg("\nProcess killed by signal: %s\n\n", sigmsg);
 
+	if (!M_CheckParm("-dedicated"))
 		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
 			"Process killed by signal",
 			sigmsg, NULL);
@@ -2202,6 +2203,7 @@ static void newsignalhandler_Warn(const char *pr)
 
 	I_OutputMsg("%s\n", text);
 
+	if (!M_CheckParm("-dedicated"))
 		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
 			"Startup error",
 			text, NULL);
@@ -2405,6 +2407,7 @@ 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
+			if (!M_CheckParm("-dedicated"))
 				SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
 					"SRB2 "VERSIONSTRING" Recursive Error",
 					buffer, NULL);
@@ -2449,6 +2452,7 @@ 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
+	if (!M_CheckParm("-dedicated"))
 		SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
 			"SRB2 "VERSIONSTRING" Error",
 			buffer, NULL);