Make initdb's version of this logic look as much like the backend's
as possible.  This is much less critical than in the backend since not
so many people use "initdb -S", but we want the same corner-case error
handling in both cases.

Back-patch to 9.3 where initdb -S option was introduced.  Before that,
initdb only had to deal with freshly-created data directories, wherein
no failures should be expected.

Abhijit Menon-Sen