Fix validation in PutBucketNotification handler (#4841)

Fixes #4813

If a TopicConfiguration element or CloudFunction element is found in
configuration submitted to PutBucketNotification API, an BadRequest
error is returned.

cmd/api-errors.go

@@ -129,6 +129,7 @@ const (
 	ErrFilterNameSuffix
 	ErrFilterValueInvalid
 	ErrOverlappingConfigs
+	ErrUnsupportedNotification
 
 	// S3 extended errors.
 	ErrContentSHA256Mismatch
@@ -552,6 +553,11 @@ var errorCodeResponse = map[APIErrorCode]APIError{
 		Description:    "Configurations overlap. Configurations on the same bucket cannot share a common event type.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrUnsupportedNotification: {
+		Code:           "UnsupportedNotification",
+		Description:    "Minio server does not support Topic or Cloud Function based notifications.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrInvalidCopyPartRange: {
 		Code:           "InvalidArgument",
 		Description:    "The x-amz-copy-source-range value must be of the form bytes=first-last where first and last are the zero-based offsets of the first and last bytes to copy",