heal: Add healing support for bucket, bucket metadata files. (#3252)

This patch implements healing in general but it is only used
as part of quickHeal().

Fixes #3237

cmd/web-handlers.go

@@ -272,15 +272,12 @@ func (web *webAPIHandlers) RemoveObject(r *http.Request, args *RemoveObjectArgs,
 		return &json2.Error{Message: errAuthentication.Error()}
 	}
 	if err := objectAPI.DeleteObject(args.BucketName, args.ObjectName); err != nil {
-		objErr := errorCause(err)
-		switch objErr.(type) {
-		case ObjectNotFound:
+		if isErrObjectNotFound(err) {
 			// Ignore object not found error.
 			reply.UIVersion = miniobrowser.UIVersion
 			return nil
-		default:
-			return &json2.Error{Message: err.Error()}
 		}
+		return &json2.Error{Message: err.Error()}
 	}
 
 	// Notify object deleted event.