Improve Verbosity of queryNamespaceMappings Errors

Previously if any errors were encountered by queryNamespaceMappings, only a count of those errors was returned - making debugging those errors harder than it needs to be.

I'm changing this to immediately return nil if no errors are encountered, and otherwise an error will be formatted with each of the namespaces and what the error was for that namespace.

Author: Jonathan Bowe

cmd/postgres_exporter/server.go

@@ -124,8 +124,11 @@ func (s *Server) Scrape(ch chan<- prometheus.Metric, disableSettingsMetrics bool
 	}
 
 	errMap := queryNamespaceMappings(ch, s)
-	if len(errMap) > 0 {
-		err = fmt.Errorf("queryNamespaceMappings returned %d errors", len(errMap))
+	if len(errMap) == 0 {
+		return nil
+	}
+	for namespace, errStr := range errMap {
+		err = fmt.Errorf("%s\nqueryNamespaceMappings error for the %s namespace: %s", err, namespace, errStr)
 	}
 
 	return err