Add warning when file to upload is too big

[seylerius]

I'm testing out Lockee.me, and on Firefox, the "Lock" button never enables, even when I type a passphrase. On Chromium, however, it enables immediately. If I manually edit the state of the button to enable it, I can upload files in Firefox just fine, however.

[silentboatman]

The lock button works fine for me. I'm running firefox 38.0.1 on Arch (x86_64). Do you have any addons enabled?

[seylerius]

Might've been an attempted upload of too large a file, looking back on the issue.

[hbons]

This should have a warning when that's the case. :)