20.04 Beta - grub installer fails and leaves system unusable

[Barnaby Dalton]

When doing a fresh installation of Ubuntu 20.04 Beta alongside Windows 10,
I get an error message during grub-installer that grub2 could not be installed.
No further details were offered, and I could not access the console after the
error was reported.
After rebooting, I am unable to access the installed system.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.9
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu22
Architecture: amd64
CasperVersion: 1.442
Date: Thu Apr 9 23:23:46 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed quiet splash ---
LiveMediaBuild: Ubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200402)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)