Improve error message of unexpected colon or comma in json macro

These rules regressed when the unimplemented macro began accepting
formatter arguments.

src/macros.rs

@@ -216,13 +216,13 @@ macro_rules! json_internal {
     // Misplaced colon. Trigger a reasonable error message.
     (@object $object:ident () (: $($rest:tt)*) ($colon:tt $($copy:tt)*)) => {
         // Takes no arguments so "no rules expected the token `:`".
-        unimplemented!($colon);
+        json_unexpected!($colon);
     };
 
     // Found a comma inside a key. Trigger a reasonable error message.
     (@object $object:ident ($($key:tt)*) (, $($rest:tt)*) ($comma:tt $($copy:tt)*)) => {
         // Takes no arguments so "no rules expected the token `,`".
-        unimplemented!($comma);
+        json_unexpected!($comma);
     };
 
     // Key is fully parenthesized. This avoids clippy double_parens false
@@ -280,3 +280,9 @@ macro_rules! json_internal {
         $crate::to_value(&$other).unwrap()
     };
 }
+
+#[macro_export]
+#[doc(hidden)]
+macro_rules! json_unexpected {
+    () => {};
+}

tests/ui/unexpected_colon.stderr

@@ -1,8 +1,8 @@
-error: expected expression, found `:`
+error: no rules expected the token `:`
   --> $DIR/unexpected_colon.rs:13:13
    |
 13 |     json!({ : true });
-   |             ^ expected expression
+   |             ^
 
 error: aborting due to previous error
 

tests/ui/unexpected_comma.stderr

@@ -1,8 +1,8 @@
-error: expected expression, found `,`
+error: no rules expected the token `,`
   --> $DIR/unexpected_comma.rs:13:17
    |
 13 |     json!({ "a" , "b": true });
-   |                 ^ expected expression
+   |                 ^
 
 error: aborting due to previous error