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.

models/update.go

@@ -102,7 +102,7 @@ func PushUpdate(opts PushUpdateOptions) (err error) {
 	gitUpdate := exec.Command("git", "update-server-info")
 	gitUpdate.Dir = repoPath
 	if err = gitUpdate.Run(); err != nil {
-		return fmt.Errorf("Fail to call 'git update-server-info': %v", err)
+		return fmt.Errorf("Failed to call 'git update-server-info': %v", err)
 	}
 
 	if isDelRef {