Address points made in post-commit review of replication origins.

Andres Freund <andres@anarazel.de>

Commit: 37163e22bddb30a235c9748f49ad54d5e99db142
Author: Andres Freund <andres@anarazel.de>
Date: 2015-08-07T13:10:00Z
Releases: 9.5.0
Address points made in post-commit review of replication origins.

Amit reviewed the replication origins patch and made some good
points. Address them. This fixes typos in error messages, docs and
comments and adds a missing error check (although in a
should-never-happen scenario).

Discussion: CAA4eK1JqUBVeWWKwUmBPryFaje4190ug0y-OAUHWQ6tD83V4xg@mail.gmail.com
Backpatch: 9.5, where replication origins were introduced.

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