grub-wrapper: don't fail on reinit/refresh error

grub-wrapper: don't fail on reinit/refresh error

these can happen for example if the system
- is set up to boot using systemd-boot
- but grub updates trigger a call to "grub-install"
- and systemd-boot is not installed

in this case, "proxmox-boot-tool reinit" will fail because of the lack of
"systemd-boot", and the upgrade triggering the grub-install call would error
out.

all the error messages/warnings are still printed and hopefully noticed.

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