Add separate error message for procedure does not exist

Peter Eisentraut <peter_e@gmx.net>

Commit: 0903bbdad24a8f51b18a6a54a41bbb36ad2ceee4
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-07-07T09:17:04Z
Releases: 12.0
Add separate error message for procedure does not exist

While we probably don't want to split up all error messages into
function and procedure variants, this one is a very prominent one, so
it's helpful to be more specific here.

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