libpq: Add missing gettext markers

libpq: Add missing gettext markers

Follow-up to 87d2801d4b: That commit restored some lost error
messages, but they ended up in a place where xgettext wouldn't find
them.  Rather than elevating ENCRYPTION_NEGOTIATION_FAILED() to a
gettext trigger, it's easiest for now to put in some explicit
libpq_gettext() calls in the couple of call sites.