Improve message when reaching socket timeout during connection initial commands

When creating a connection + executing initial queries, connector set temporary a socket timeout (SO_TIMEOUT) of 500ms to avoid being stuck ( that was the case with the first version of maxscale and in all version of aurora). When this timeout error, the error label is just "could not load system variables".
This error must be improved to clearly state that this comes from reaching socket timeout.

At the same time, when under stress, this 500ms timeout is too slow. This has been changed in 3.0 with a dedicated option `connectTimeout` that default to 30s, correction will change this non modifiable option in 2.x to 30s as well.