Ensure pg_up is always set to 0 when the initial database connection fails.

This did in fact turn out to be an oversight in the error handling. Now, any
error in the initial connection path will always trip pg_up to be 0.

Fixes #160

cmd/postgres_exporter/postgres_exporter.go

@@ -969,15 +969,12 @@ func (e *Exporter) getDB(conn string) (*sql.DB, error) {
 	if e.dbConnection == nil {
 		d, err := sql.Open("postgres", conn)
 		if err != nil {
-			e.psqlUp.Set(0)
 			return nil, err
 		}
 		err = d.Ping()
 		if err != nil {
-			e.psqlUp.Set(0)
 			return nil, err
 		}
-		e.psqlUp.Set(1)
 
 		d.SetMaxOpenConns(1)
 		d.SetMaxIdleConns(1)
@@ -1011,6 +1008,7 @@ func (e *Exporter) scrape(ch chan<- prometheus.Metric) {
 		}
 		log.Infof("Error opening connection to database (%s): %s", loggableDsn, err)
 		e.error.Set(1)
+		e.psqlUp.Set(0) // Force "up" to 0 here.
 		return
 	}