Make grub-install check for errors from efibootmgr

Code is currently ignoring errors from efibootmgr, giving users
clearly bogus output like:

        Setting up grub-efi-amd64 (2.02~beta3-4) ...
        Installing for x86_64-efi platform.
        Could not delete variable: No space left on device
        Could not prepare Boot variable: No space left on device
        Installation finished. No error reported.

and then potentially unbootable systems. If efibootmgr fails, grub-install
should know that and report it!

We've been using similar patch in Debian now for some time, with no ill effects.

Signed-off-by: Steve McIntyre <93sam@debian.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/util/install.h

@@ -210,7 +210,7 @@ grub_install_create_envblk_file (const char *name);
 const char *
 grub_install_get_default_x86_platform (void);
 
-void
+int
 grub_install_register_efi (grub_device_t efidir_grub_dev,
 			   const char *efifile_path,
 			   const char *efi_distributor);