Enhance healthcheck for database connectivity (#3754)

Modify the call to ping the database to fail gracefully if the
database has not yet been configured by the end user, such as
after a clean install. This allows /healthcheck to return a 200
with a modified status message instead of causing a PANIC.

Signed-off-by: Evan Sherwood <evan@sherwood.io>

models/models.go

@@ -335,7 +335,10 @@ func GetStatistic() (stats Statistic) {
 
 // Ping tests if database is alive
 func Ping() error {
-	return x.Ping()
+	if x != nil {
+		return x.Ping()
+	}
+	return errors.New("database not configured")
 }
 
 // DumpDatabase dumps all data from database according the special database SQL syntax to file system.