File drop GUI: unclear feedback to user when file is too large

[wobbieh]

Inside the demo (http://demo.hub3.delving.eu/narthex/) when dragging the XML file (linked below) into the 'File Drop' we get an undefined error - likely a software bug. The data doesn't appear in Narthex.

.sip.zip files don't cause this error.

file: https://drive.google.com/file/d/0B_tagvypJifYczdSQ21Wdk1lU1U/view?usp=sharing
(Too big for uploading here)

[hanswesterbeek]

The filesize was also the issue for narthex. To prevent this from happening, the GUI should provide better feedback to the user about the maximum file-upload size, so they won't be left in this kind confusion

[hanswesterbeek]

Postponed this because setting the value of play's max_upload_size to 50mb (like we have) will have made this issue go away in most cases.