Refactor warning diagnostic code to reduce repeated output logic

src/usage.cpp

@@ -6,6 +6,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#include "diagnostics.hpp" // vfatalx
 #include "helpers.hpp"
 #include "platform.hpp"
 #include "style.hpp"
@@ -156,13 +157,9 @@ void Usage::printAndExit(int code) const {
 
 void Usage::printAndExit(char const *fmt, ...) const {
 	va_list args;
-	style_Set(stderr, STYLE_RED, true);
-	fputs("FATAL: ", stderr);
-	style_Reset(stderr);
 	va_start(args, fmt);
-	vfprintf(stderr, fmt, args);
+	vfatalx(fmt, args);
 	va_end(args);
-	putc('\n', stderr);
 
 	printAndExit(1);
 }