FWIW the log during the latest upgrade attempt was a bit more verbose, here it is:
Setting up shim-signed (1.28+nmu3+0.9+1474479173.6c180c6-1) ... Installing for x86_64-efi platform. grub-install: warning: efivarfs_get_variable: open(/sys/firmware/efi/efivars/blk0-47c7b225-c42a-11d2-8e57-00a0c969723b): No such file or directory. grub-install: warning: efi_get_variable: ops->get_variable failed: No such file or directory. grub-install: warning: is_gpt_valid: GUID Partition Table Header signature is wrong: 8e7c00bcd08ec033 != 5452415020494645: Invalid argument. grub-install: warning: is_gpt_valid: GUID Partition Table Header signature is wrong: 4441454856495250 != 5452415020494645: Invalid argument. grub-install: warning: msdos_disk_get_extended_partition_info: extended partition info is not supported: Function not implemented. grub-install: warning: msdos_disk_get_partition_info: could not get extended partition info: Function not implemented. grub-install: warning: get_partition_info: neither MBR nor GPT is valid: Function not implemented. grub-install: warning: make_hd_dn: could not get partition info: Function not implemented. grub-install: warning: efi_va_generate_file_device_path_from_esp: could not make HD() DP node: Function not implemented. grub-install: warning: efi_generate_file_device_path_from_esp: could not generate File DP from ESP: Function not implemented. grub-install: error: failed to register the EFI boot entry: Function not implemented. dpkg: error processing package shim-signed (--configure): installed shim-signed package post-installation script subprocess returned error exit status 1 Errors were encountered while processing: shim-signed
