Gracefully handle when there are no Postgres errors from the db sync
Gracefully handle when there are no Postgres errors from the db sync

- We're so used to Postgres throwing errors, that we didn't account for
  the case where everything is fine, so we were seeing new data sync
  failures like:

  ```
  govuk_env_sync.sh: Starting "/usr/local/bin/govuk_env_sync.sh -f /etc/govuk_env_sync/pull_email-alert-monitor_production_daily.cfg"
download: s3://govuk-staging-database-backups/postgresql-backend/2018-09-14T00:45:01-email-alert-monitor_production.gz to ../../tmp/email-alert-monitor_production/tmp.3K6KtWa73M/2018-09-14T00:45:01-email-alert-monitor_production.gz
/usr/local/bin/govuk_env_sync.sh: line 66: pg_errors[@]: unbound variable
  ```

- Instead, wrap the error calculation in an if for the "not empty"
  presence of the error variable.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet