Remove the escape backslash from the result string.

src/error.c

@@ -249,8 +249,15 @@ mrb_vformat(mrb_state *mrb, const char *format, va_list ap)
       }
     }
     else if (c == '\\') {
-      if (!*p) break;
-      p++;
+      if (*p) {
+        size = p - b - 1;
+        mrb_ary_push(mrb, ary, mrb_str_new(mrb, b, size));
+        mrb_ary_push(mrb, ary, mrb_str_new(mrb, p, 1));
+        b = ++p;
+      }
+      else {
+        break;
+      }
     }
     p++;
   }