Raise exception or some sort of message when user tries to upload file larger than max file limit

[dandanxu]

Right now you get a AWS timeout, which might be confusing to the enduser

looks like requests.exceptions.ProxyError: HTTPSConnectionPool(host='solvebio-secure-production.s3.amazonaws.com', port=443): Max retries exceeded with url: ... (Caused by ProxyError('Cannot connect to proxy.', error("(32, 'EPIPE')",)))

[jsh2134]

Linked to #165