More urgent warnings

[nylen]

https://forums.classicpress.net/t/new-install-failed/3543

Make it even more clear that having a backup is not sufficient - you must be ready to restore it because the migration process is not and never will be 100% foolproof. This is fairly obvious to me – and pretty heavily implied by phrases such as “just in case something does not go as planned” – but apparently not good enough. As part of this change we can also make it clear that we check for known incompatibilities with plugins and themes, but we can’t check for all the unknown incompatibilities.