Report a response body over the limit as ExceedMaxPayloadSize

A buffered request whose response body went over set_payload_max_length(),
also once decompressed, failed with Error::Read, while the Stream API
reports the same limit as Error::ExceedMaxPayloadSize. read_content()
already marks it with a 413 status, so map that status to
ExceedMaxPayloadSize next to the 415 and 500 the client maps already.

README.md

@@ -1268,6 +1268,8 @@ cli.set_max_timeout(std::chrono::seconds(5));
 cli.set_payload_max_length(1024 * 1024 * 512); // 512MB
 ```
 
+A response body over the limit, also once decompressed, fails the request with `Error::ExceedMaxPayloadSize`.
+
 ### Receive content with a content receiver
 
 ```c++