sock's method: connect, async_connect throw different exception.

[Eggache666]

When using sock's method, 'connect' and 'async_connect' to connect to a server that does not exist, the exception thrown is different: (boost::system::system_error)

the message of e.what():
'connect": "connect: Connection refused"
'async_connect': "Connection refused"

I think it is better to output the same information.

platform: ubuntu18.10, boost1.68, clang7.0.0

This issue was moved by chriskohlhoff to chriskohlhoff/asio#652.