Catch errors when uploading files

[ikisler]

If you attempt to upload a file that is too large, or some other upload error occurs (file only uploads partially, etc.), it gives you an "unkown file type" error, rather than something more descriptive. Added a message specifically for file too large, and another general one if something else in the upload fails.

[sbrl]

I think you included this in your other PR! I'll do them both at once, sincce I've already edited the other once :-)