Warn user when upload file size exceeds limit

[I-Valchev]

OK currently if the file size is above the min of php.ini's upload_max_filesize, post_max_size or the fileUploader rule in Bolt, the server rejects it, but there is no adequate feedback on the frontend.

What should happen instead is to limit the max file size on the frontend as well, so that the user has some sort of feedback that they are trying to do something that Bolt is not allowed to do with the present configs.