Use error handler to report errors

test/format-test.cc

@@ -1906,4 +1906,7 @@ TEST(FormatTest, FormatStringErrors) {
   EXPECT_ERROR("{1}{}",
                "cannot switch from manual to automatic argument indexing",
                int, int);
+  EXPECT_ERROR("{}{1}",
+               "cannot switch from automatic to manual argument indexing",
+               int, int);
 }

test/printf-test.cc

@@ -116,7 +116,7 @@ TEST(PrintfTest, InvalidArgIndex) {
       format_error, "argument index out of range");
 
   EXPECT_THROW_MSG(fmt::sprintf("%2$", 42),
-      format_error, "invalid format string");
+      format_error, "argument index out of range");
   EXPECT_THROW_MSG(fmt::sprintf(format("%{}$d", BIG_NUM), 42),
       format_error, "number is too big");
 }