[MNG-6891] Improve user-friendliness --fail-on-severity

[mthmulders]

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[michael-o]

Looks incomplete, what about? ERROR(ERROR_INT, "ERROR"), WARN(WARN_INT, "WARN"), INFO(INFO_INT, "INFO"), DEBUG(DEBUG_INT, "DEBUG"), TRACE(TRACE_INT, "TRACE");

[mthmulders]

This merge request is only addressing the first part of MNG-6891. I'm planning to do the second part (support WARNING as well) in a follow-up.

[mthmulders]

On second thought, I'll add the remainder of MNG-6891 to this merge request, too. That is, the part that says "it'd be nice if WARNING was accepted next to WARN".

Since this feature hasn't been released yet, I took the liberty to change the message for severity levels that we don't accept for --fos. Instead of "Logging severity thresholds can only be set to WARN or ERROR" it'll now give the same message "INFO is not a valid log severity threshold. Valid severities are WARN/WARNING and ERROR.".

This shouldn't break anything, as the feature hasn't been released yet, but makes the code a little easier to read, and it saves us one try/catch.

[2997ms]

Then this won't be executed, I make a pr to delete this. #360