Bad UX with "Connection closed" errors when downloading support files

[pkgw]

When a fresh Tectonic install is downloading support files, I find that often you end up with an error because one of the HTTP connections gets dropped:

note: downloading cmr8.tfm
note: downloading cmmi8.tfm
warning: open of input cmmi8 failed
caused by: Connection closed
caused by: Connection closed
error: something bad happened inside TeX; its output follows:

This is bad for two reasons. First, Tectonic then dumps the engine output and ends with an error message saying

error: halted on potentially-recoverable error as specified

This scrolls the real problem off of the screen and will confuse the user.

Second, this happens often enough that we should deal with it more gracefully. The obvious thing to do is just to retry downloads a few times if they error out.