s3: report error to s3 when updating an object but it is already a directory

fix https://github.com/chrislusf/seaweedfs/issues/1545

weed/s3api/s3err/s3api_errors.go

@@ -92,6 +92,8 @@ const (
 	ErrMissingDateHeader
 	ErrInvalidRequest
 	ErrNotImplemented
+
+	ErrExistingObjectIsDirectory
 )
 
 // error code to APIError structure, these fields carry respective
@@ -344,6 +346,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "A header you provided implies functionality that is not implemented",
 		HTTPStatusCode: http.StatusNotImplemented,
 	},
+	ErrExistingObjectIsDirectory: {
+		Code:           "ExistingObjectIsDirectory",
+		Description:    "Existing Object is a directory.",
+		HTTPStatusCode: http.StatusConflict,
+	},
 }
 
 // GetAPIError provides API Error for input API error code.