Make scroll-{up,down} go all the way.
Out of the box GNU Emacs only scrolls the buffer when using scroll-down
and scroll-up (which most people would have bound to PgDn and
PgUp). Once the first or last lines are visible you'll get
start/end of buffer errors if you try and hit those keys more. Many other
editors will, when that happens, move the cursor to the start or end of the
buffer. This code makes GNU Emacs do that too.