Add error log when probe collector creation fails (prometheus-community#918)

Signed-off-by: Joe Adams <github@joeadams.io>

Author: sysadmind

cmd/postgres_exporter/probe.go

@@ -85,6 +85,7 @@ func handleProbe(logger log.Logger, excludeDatabases []string) http.HandlerFunc
 // Run the probe
 pc, err := collector.NewProbeCollector(tl, excludeDatabases, registry, dsn)
 if err != nil {
+level.Error(logger).Log("msg", "Error creating probe collector", "err", err)
 http.Error(w, err.Error(), http.StatusInternalServerError)
 return
 }