If you leave a host in build mode, but after a while either DNS in the Foreman is broken, or the host is not available any more for whatever reason, cancelling the build becomes impossible.
It raises a 500 error with 'getaddrinfo: Name or service not known' or any similar exception, depending on the problem.
I think this should be handled gracefully and displayed to the user in a way the user can know what to do with this host now (check DNS, check the compute resource is reachable, etc...).