Communicate when requests fail because of a firewall

[adamziel]

universities and enterprises often have network level firewalls that reject some some of the network activity playground needs. This often leads to confusing network errors like 404, 500, 404. or let's detect whenever those failures happen for reasons different than the remote URL actually responding with one of these error codes. and communicate that to the user. This may happen at many stages, but so far it's been the most confusing during the blueprint execution, execution, so let's focus on that stage first. the the course proxy endpoint in playground could return a special header. And that header will tell us whether the response actually comes from the cross proxy or whether it has been hijacked by some intermediary. If we ever get a response without that header during the blueprint execution, let's display the error model explaining what just happened and what next steps the user must take to successfully execute the blueprint.