bug#22619: postgresql-service error checking wtf

[Danny Milosavljevic]

Package: postgresql

When I first configure operating-system with postgresql-service with default 
config file, then run

guix system reconfigure config.scm,

then configure operating-system with postgresql-service with custom config 
file, then run

guix system reconfigure config.scm,

I get:

---
initdb: directory "/var/lib/postgresql/data" exists but is not empty
If you want to create a new database system, either remove or empty
the directory "/var/lib/postgresql/data" or run initdb
with an argument other than "/var/lib/postgresql/data".
making '/gnu/store/byn534fg4agm0vv9szl5kkfl19n080b2-system' the current 
system...
Installation finished. No error reported.
----

Postgresql doesn't run and the error message still references the default 
config file.