Better error messages when BOOST_ASIO_CONCURRENCY_HINT_UNSAFE is set

[mzimbres]

We should either support BOOST_ASIO_CONCURRENCY_HINT_UNSAFE or improve the error message when it is set (or any other hint that prevents async_resolve). At the moment constructing an io_context with BOOST_ASIO_CONCURRENCY_HINT_UNSAFE produces the following error messages

(Boost.Redis) Trying to connect to Redis server at 127.0.0.1:6379 (TLS disabled)
(Boost.Redis) Connect: hostname resolution failed: Operation not supported [system:95]
(Boost.Redis) Failed to connect to Redis server at 127.0.0.1:6379 (TLS disabled): Operation not supported [system:95]
(Boost.Redis) Trying to connect to Redis server at 127.0.0.1:6379 (TLS disabled)
(Boost.Redis) Connect: hostname resolution failed: Operation not supported [system:95]
(Boost.Redis) Failed to connect to Redis server at 127.0.0.1:6379 (TLS disabled): Operation not supported [system:95]
(Boost.Redis) Trying to connect to Redis server at 127.0.0.1:6379 (TLS disabled)
(Boost.Redis) Connect: hostname resolution failed: Operation not supported [system:95]
(Boost.Redis) Failed to connect to Redis server at 127.0.0.1:6379 (TLS disabled): Operation not supported [system:95]