add s3api error for copy in file, not directory

weed/s3api/s3err/s3api_errors.go

@@ -101,6 +101,7 @@ const (
 	ErrPreconditionFailed
 
 	ErrExistingObjectIsDirectory
+	ErrExistingObjectIsFile
 )
 
 // error code to APIError structure, these fields carry respective
@@ -383,6 +384,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "Existing Object is a directory.",
 		HTTPStatusCode: http.StatusConflict,
 	},
+	ErrExistingObjectIsFile: {
+		Code:           "ExistingObjectIsFile",
+		Description:    "Existing Object is a file.",
+		HTTPStatusCode: http.StatusConflict,
+	},
 }
 
 // GetAPIError provides API Error for input API error code.