handle bootloader installation errors gracefully

handle bootloader installation errors gracefully

by collecting them, but proceeding with the rest of the installation. it
is possible that only either legacy or EFI boot fails, and even if both
do, manually fixing the bootloader setup by booting a live CD is very
often possible.

still display the error screen at the end if bootloader setup was not
successful, so that expectations are not set too high..

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>