Change log dumping default and make it configurable

[cliffano]

Currently Stack Manager Messenger dumps the log file only when there is a failure.
However, many users prefer to always have the log file regardless, which seems to be a reasonable default, i.e. users always want to see log output to 1) identify what caused a failure, 2) verify that a success is doing what is expected .

We need to do the following:

1. Change default behaviour to always dump the log
2. Introduce configuration show_log_on_failure_only , and set it to false by default

This configuration is handy for users who want to dump the log file only on failure.

[cliffano]

Done. Included in v1.3.0 .