Improve error handling when server response can't be accommodated in the response object

For example

```cpp
request req;
req.push("ping", "abc");
req.push("ping", "def");

response res;

co_await conn.async_exec(req, res);
```
Allowing internal adapter code is wrong, the library should communicate an error with which the async operation will complete.

Note: This touches only responses with an static size i.e. `response` and not the `generic_response`.