Fail unresponded requests with a different error code

We offer `config::cancel_if_unresponded`, which makes a request fail if it was sent to the server but never responded because the connection was severed. There is no way to know whether these requests were processed by the server or not. At the moment, `async_exec` completes with `asio::operation_aborted` for these requests. This makes it impossible for the user to know when this situation happened. We should create a new error code for this situation. See #324 for the discussion.

Start by examining config::cancel_if_unresponded and the async_exec completion path that currently returns asio::operation_aborted. Read the discussion in #324 before deciding how the new error code should represent an unresponded request. Done means the situation has a distinct error code and the affected behavior is covered by the relevant tests.

Written by the indexing model from the issue text.