Refactor FileTransfer.afterSuccess to afterTransfer

In order to prevent the deleting of files when MoveFileTransfer
**fails**, it was necessary to provide the callback method with
the number of errors.

Further, to have the number of errors properly reported, it was
necessary to notify of a FILE_EXCEPTION on any exception during
uploadFile.

Depending on other import settings, this could trigger a file
upload, which in the case of in-place import is to be avaoided.

/cc @mtbc @pwalczysko