UX: Direct the user to manual installation methods when the installer fails

[coderbot16]

Sometimes, there are intermittent issues with the services that the Iris installer relies on, or firewall issues, or any number of other difficult-to-pinpoint problems. When the installer fails, users don't realize that it isn't the only way to install the mod, and falsely believe that they don't have another way to install Iris. We should direct users to https://github.com/IrisShaders/Iris/blob/1.18.2/docs/guide.md#manual-installation in error messages so that they can still have a clear way to use the mod.