MINOR: connection: add more error codes to report connection errors

It is quite often that an connection error only reports "socket error" with
no more information. This is especially problematic with health checks where
many causes are possible, including resource exhaustion which do not lead to
a valid errno code. So let's add explicit codes to cover these cases.
MEDIUM: tcp: report connection error at the connection level

Now when a connection error happens, it is reported in the connection
so that upper layers know exactly what happened. This is particularly
useful with health checks and resources exhaustion.
MEDIUM: checks: make use of chk_report_conn_err() for connection errors

Checks used not to precisely report the errors that were detected at the
connection layer (eg: too many SSL connections). Using chk_report_conn_err()
makes this possible.