Add detection of SSL problems on startup

[drojf]

Currently we download a file from 07th-mod website to check connectivity, but we don't fallback to downloading via http if https fails.

It may be a good idea to try both - if https fails but http succeeds, likely there is some SSL error, and a warning can be shown in the installer, and a summary of probe results can be printed to the log.

Also, probing both github and 07th-mod.com would be a good idea, as both are required to install the mod.

Also see #189