Allow ScribbleDecoder to set severity from the message template.

Closes #1084.
I left it in b/c I have a vague memory that there was a reason it was put there in the first place. Unfortunately that was well over a year ago and I don't remember the specifics. I'm willing to remove it, but not in this PR which was opened against versions/0.7, since (small as it is) it's technically a breaking change.