Handling of failed configure requirements

Start of discussion at https://github.com/haskell/ghcup/issues/8#issuecomment-434270296

Also, handling this via duplicating configure script dependency checking feels a bit odd to me. Next thing, someone wants the same for ghcup compile... and we end up doing half-baked toolchain checking.

Any chance the issues in the haskell/ghcup repo could be re-enabled, with perhaps https://probot.github.io/apps/lock/ installed so it locks all existing issue and pull request threads (which I suspect is why they were disabled)?

@dwijnand why?

To see https://github.com/haskell/ghcup/issues/8#issuecomment-434270296 and other similar issue/PR threads that are now not visible. Ideally one could make all issues read-only without hiding them.

All issues are visible here. I don't understand what you mean.

Ah, sorry. I hadn't realised those were also copied over, with matching issue numbers. The comment identifier fragement isn't usable, but that's minor. Sorry for the noise.