I don't know a good solution for logging the errors.
But a complicated workaround:

Doin't boot into X11, but to console (/etc/inittab: id:4:initdefault instead of id:5:...).
With Shift-PageUp you may scroll to the next page of messages.

Perhaps someone has a logfile-solution.

I don't know whether Ctrl-S (stop) and Ctrl-Q (resume) works during bootup, to stop the output. At least you may try.