Until now, if the .sql file contained invalid data, psql would
choke on it, but still return an exit code of 0.
This is very misleading.

We need to pass `-v ON_ERROR_STOP=1` to make it exit
with a proper error exit code when failures happen.

We've had that logic in 2 places so far, leading to duplication
and a maintenance burden.

In the future, we'll also have an import-postgres feature,
which will also need Postgres version detection,
leading to more benefit from that logic being reusable.

This also raises the default from 5 seconds to 15 seconds.

Regression since 3fd6fd647f

Pretty much all variables live in their own `matrix_<whatever>`
prefix now and are grouped closer together in the default
variables file (`roles/matrix-server/defaults/main.yml`).

Since cbee084ac19b6, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.

This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).