Bug#866758: solved (maybe)

[Thomas Troeger <tstroege@gmx.de>]

Currently X will start again. As it seems, the modules for nVidia needed
kernel headers to compile, which were not available for kernel 4.9.0-3.
Changing kernel and header by hand to 4.11.0-1-amd64 and forcing a
recompile of the nVidia kernel modules solved the issue -- at least
temporarily.

I think it is very important that, on error, the X server tries to boot
into a default X server that works for all graphics adapters (vesa ...
?) so you get a graphical user interface at all. Text mode as fallback,
especially in 80x24, is a little heavy-handed for all those
inexperienced users out there.