translatability tweaks

Here's a small number of translatability tweaks to the error messages in
the backend.  I found these while updating the Spanish translation over
the past weekend, a task I had neglected for two or three years, so they
might involve some older messages.  However, I won't backpatch this --
only apply to pg12 tomorrow or so.

(I haven't yet gone over the full backend message catalog yet, so I
might propose more fixes later on.)

-- 
Álvaro Herrera                                http://www.twitter.com/alvherre