Better error handling for submit failure in CloudCare

The error message for a cross-site script error looks something like this (assuming the user loaded the site from http://localhost:8080/... and localsettings.py has BASE_ADDRESS = "127.0.0.1:8080"):

Unknown error: error 0. This can happen if you loaded CloudCare from a different address than the server address (http://127.0.0.1:8080/a/test/receiver/6717bd0cf4ada93d80ed2a41b67dee6b/)

NOTE I’m not sure why cloudcare has to use an address provided by the server. Why not submit to a path like /a/test/receiver/6717bd0cf4ada93d80ed2a41b67dee6b/ rather than the fully qualified URL?
Merge pull request #4209 from dimagi/better-cloudcare-error

Better error handling for submit failure in CloudCare