Cleanup log messaging

This change corrects a few logging issues:

 * Standardized formatting errors with '%v'.
 * Standardized failure warning word usage.
 * Corrected an instance of using the standard log library when
   the gitea log library should be used instead.

cmd/web_graceful.go

@@ -8,9 +8,9 @@ package cmd
 
 import (
 	"crypto/tls"
-	"log"
 	"net/http"
 
+	"code.gitea.io/gitea/modules/log"
 	"github.com/facebookgo/grace/gracehttp"
 )