Be more verbose on failed connection

[renecannao]

When a connection to a backed fails, proxysql returns an error similar to:
"Standard_MySQL_Thread.cpp:1099:myds_backend_pause_connect(): connect() error on fd 14 . Pausing ..."

This is not extremely useful for debugging, and more information should be returned in the error. At least, the address of the backend should be reported

[shahsavar63mhnztalebi]

I had this problem because I use default proxysql.cfg file in src directory. In this file, multiple mysql servers have been introduced to proxysql, but I just have one mysql server (127.0.0.1:3306).