Add user-friendly error messages when uploading large files

[mccc]

When uploading files > 10 MB, one gets either a "413 Request Entity Too Large" nginx error or geomancer simply hangs "forever". (I reproduced the former with a 100+MB .xls file and the latter with a 40 MB .xlsx file.) These should be changed to a more user-friendly error message if possible.

[evz]

I have yet to find a decent solution for this. There are several nuances with the way that Flask and nginx interact that make it somewhat tricky to pin down. I'll keep looking but I'm punting this for now.