anchor.diff:                                             query.NativeBson, @err,
                                              FlagsToNative(AFlags));
     if nativeFile = nil then
  -    raise EMongoGridfs.Create('File not found');
  +    RaiseFindFileError(@err);
   
     Result := NewMongoGridfsFile(nativeFile);
   end;
  
positive.code:   err: bson_error_t;
    query: IBson;
  begin
    if AQuery = nil then
      query := NewBson
    else
  
negative.code:     FNativeGridfs: Pointer;
      FError: bson_error_t;
      function FlagsToNative(AFlags: TMongoFlags): Integer;
      procedure RaiseFindFileError(err: bson_error_p);
    public
      constructor Create(ANativeGridfs: Pointer);
      destructor Destroy; override;
  