printf: Show which format specifier is unknown

libs/libc/src/printf.cpp

@@ -4,6 +4,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include <sys/syscall.h>
 #include <unistd.h>
@@ -237,7 +238,11 @@ static int internal_printf(const char* format, PutString put_string_callback, ss
                     break;
                 }
                 default: {
-                    NOT_IMPLEMENTED("internal_printf: unknown format specifier");
+                    fputs("printf: unknown format specifier ", stderr);
+                    fputc('%', stderr);
+                    fputc(current_char, stderr);
+                    fputc('\n', stderr);
+                    abort();
                 }
                 }
             }