upgrade error 16.04 ubuntu

tried to upgrade from terminal, but getting error message saying

henry@polaris:~$ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
4 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Setting up linux-image-4.4.0-24-generic (4.4.0-24.43) ...
Running depmod.
update-initramfs: deferring update (hook will be called later)
initrd.img(/boot/initrd.img-4.4.0-24-generic
) points to /boot/initrd.img-4.4.0-24-generic
 (/boot/initrd.img-4.4.0-24-generic) -- doing nothing at /var/lib/dpkg/info/linux-image-4.4.0-24-generic.postinst line 491.
vmlinuz(/boot/vmlinuz-4.4.0-24-generic
) points to /boot/vmlinuz-4.4.0-24-generic
 (/boot/vmlinuz-4.4.0-24-generic) -- doing nothing at /var/lib/dpkg/info/linux-image-4.4.0-24-generic.postinst line 491.
Examining /etc/kernel/postinst.d.
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/dkms 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
update-initramfs: Generating /boot/initrd.img-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/pm-utils 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/unattended-upgrades 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 4.4.0-24-generic /boot/vmlinuz-4.4.0-24-generic
Searching for GRUB installation directory ... found: /boot/grub
/etc/default/grub: line 20: syntax error near unexpected token `grub-pc'
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 2
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-4.4.0-24-generic.postinst line 1052.
dpkg: error processing package linux-image-4.4.0-24-generic (--configure):
 subprocess installed post-installation script returned error exit status 2
dpkg: dependency problems prevent configuration of linux-image-extra-4.4.0-24-generic:
 linux-image-extra-4.4.0-24-generic depends on linux-image-4.4.0-24-generic; however:
  Package linux-image-4.4.0-24-generic is not configured yet.

dpkg: error processing package linux-image-extra-4.4.0-24-generic (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent configuration of linux-image-generic:
 linux-image-generic depends on linux-image-4.4.0-24-generic; however:
  Package linux-image-4.4.0-24-generic is not configured yet.
 linux-image-generic depends on linux-image-extra-4.4.0-24-generic; however:
  Package linux-image-extra-4.4.0-24-generic is not configured yet.

dpkg: error processing package linux-image-generic (--configure):
 dependency problems - leaving unconfigured
dpkg: dependency problems prevent cNo apport report written because the error message indicates its a followup error from a previous failure.
                                                             No apport report written because the error message indicates its a followup error from a previous failure.
       No apport report written because MaxReports is reached already
                                                                     onfiguration of linux-generic:
 linux-generic depends on linux-image-generic (= 4.4.0.24.25); however:
  Package linux-image-generic is not configured yet.

dpkg: error processing package linux-generic (--configure):
 dependency problems - leaving unconfigured
Errors were encountered while processing:
 linux-image-4.4.0-24-generic
 linux-image-extra-4.4.0-24-generic
 linux-image-generic
 linux-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)