Bug#843113: better missing kernel headers message

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

Package: src:linux
Version: 4.8.5-1
Severity: wishlist

Setting up linux-image-4.8.0-1-686-pae (4.8.5-1) ...
I: /vmlinuz.old is now a symlink to boot/vmlinuz-4.7.0-1-686-pae
I: /initrd.img.old is now a symlink to boot/initrd.img-4.7.0-1-686-pae
I: /vmlinuz is now a symlink to boot/vmlinuz-4.8.0-1-686-pae
I: /initrd.img is now a symlink to boot/initrd.img-4.8.0-1-686-pae
/etc/kernel/postinst.d/dkms:
Error! Your kernel headers for kernel 4.8.0-1-686-pae cannot be found.
Please install the linux-headers-4.8.0-1-686-pae package,
or use the --kernelsourcedir option to tell DKMS where it's located

1. That error message is not coming from that file. In fact from no
file with dkms in its name:
$ dlocate --filename-only dkms|xargs grep Error.*You 2>&-|wc -l
0

2. Please also have the message mention if one then needs to reinstall
linux-image-4.8.0-1-686-pae .

(Seen when offine installing from a /var/cache/apt/archives/ that has
some, but not all packages...)