Poor support for upload_max_filesize

[vrana]

If the size of an uploaded file exceeds PHP's upload_max_filesize then the form field reports just a generic error not mentioning size at all. I suggest two improvements:

1. If the size set in Form::MAX_FILE_SIZE exceeds upload_max_filesize then throw an exception.
2. If the file upload fails with UPLOAD_ERR_INI_SIZE, report the same error as when exceeding Form::MAX_FILE_SIZE.

There are already some checks for post_max_size in the code, maybe something similar could be done for upload_max_filesize.