core: improve file cancel (#627)

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

src/Simplex/Chat/Controller.hs

@@ -335,6 +335,7 @@ data ChatErrorType
   | CEGroupInternal {message :: String}
   | CEFileNotFound {message :: String}
   | CEFileAlreadyReceiving {message :: String}
+  | CEFileCancelled {message :: String}
   | CEFileAlreadyExists {filePath :: FilePath}
   | CEFileRead {filePath :: FilePath, message :: String}
   | CEFileWrite {filePath :: FilePath, message :: String}