HTTP API: its own 400 for a query that cannot be read (closes #5)

GET /api/v1/chats/{id}/messages answered any query that does not
decode, such as one holding ; or %zz, with the sentence about count,
even when count was valid or absent. It now answers such a query with
"the query cannot be read", and keeps the count sentence for a count
that decodes but is not a whole number from 1 to 100. The README names
both cases.

Model: opus-5-5

README.md

@@ -179,8 +179,10 @@ A message has:
   the SimpleX relay, to the second; for a sent one, when the bot sent
   it.
 
-A `count` other than a whole number from 1 to 100 gets `400`, and an
-`id` that `GET /api/v1/chats` does not list gets `404`.
+The answer is `400` if the query cannot be read (it holds a `;`, or a
+`%` not followed by two hexadecimal digits) or `count` is not a whole
+number from 1 to 100, and `404` if `GET /api/v1/chats` does not list
+`id`.
 
 ### `POST /api/v1/chats/{id}/messages`