failures in `initdb` are not reported

[danbornside]

While trying to create a db instance in docker, Initdb fails with the following error, which is silently swollowed by the use of runInteractiveProcess and discarding the handle for stderr.

+ exec ./backend
initdb: cannot be run as root
Please log in (using, e.g., "su") as the (unprivileged) user that will
own the server process.

["gargoyle-postgresql::Gargoyle.PostgreSQL(initLocalPostgres)","/nix/store/phf8zhbbvfar40fz3jdbfxq9bg5l6hvp-postgresql-9.5.9/bin/initdb","db/work"]
backend: ExitFailure 1
CallStack (from HasCallStack):