Don't log on Slack errors that occur when running CLI commands

[gturri]

Errors that occur when we run the CLI commands are already seen by the person running it, it's pointless to report it on Slack.
(nb: perhaps it should be tackled not by the code in this repo, but by the local conf. In which case it would be great to document this)

[gturri]

(not sure how it can be achieved. Hopefully I'll get some answers on https://stackoverflow.com/q/78359833/1796345 )

[gturri]

If this symfony/monolog-bundle#480 can work, then we could fix this with a simple custom handler that would wrap the Slack handler, and filter out everything in a CLI context.