Fix supported distro checking and handling

[askiiart]

Previously, distros were checked in both blend and user, which led to user's distro list being forgotten about and severely out of date, not to mention incorrect in the first place; it's a blessing that the code didn't work as it probably should have - that is, by exiting if the distro was unsupported.

With this, supported distro checking is handled by blend only, and it now has an exit code of 1 if the distro is unsupported, and user returns that exit code.

[askiiart]

I also threw in a fix so that user's default distro is arch-linux, which is what blend actually uses, not arch.

Anyways, it just needs to verified that none of this will break the frontend, and then we should be good to go.

[askiiart]

Turns out no distro being specified is already handled by blend, so I updated it so that user doesn't add a distro in the case that one isn't specified.