Soften Tor log output for non critical events

We had a forum post from a user who interpreted one of the snowflake connection events as a critical failure of snowflake: https://forum.torproject.net/t/snowflake-does-not-work-anymore/2650/2

While a failure to connect to the broker can be critical, a failure to open a data channel with a snowflake is not unusual and snowflake can easily recover from it. Let's make a small change of the log message from "connection failed" to "trying a new proxy: [error message]" or something like that