When severity is set to N/A for a defect, mention it is not a valid value instead of asking to set severity

[mkaply]

See:

https://bugzilla.mozilla.org/show_bug.cgi?id=1964247#c23

I set the severity/priority yesterday, but BugBot still said the severity wasn't set.

[marco-c]

The reason is that N/A is not a valid severity for defects (https://wiki.mozilla.org/BMO/UserGuide/BugFields#bug_severity).

[mkaply]

Ah, that makes sense.

The message should probably be updated then. That's not clear to me.