Optimize alignment parsing

test/format-test.cc

@@ -646,7 +646,7 @@ TEST(FormatterTest, Fill) {
   EXPECT_EQ(std::string("\0\0\0*", 4), format(string_view("{:\0>4}", 6), '*'));
   EXPECT_EQ("жж42", format("{0:ж>4}", 42));
   EXPECT_THROW_MSG(format("{:\x80\x80\x80\x80\x80>}", 0), format_error,
-                   "invalid fill");
+                   "missing '}' in format string");
 }
 
 TEST(FormatterTest, PlusSign) {