Scroll text of selected window upward ARG lines; or near full screen if no ARG.
If `scroll-error-top-bottom' is non-nil and `scroll-up' cannot
scroll window further, move cursor to the bottom line.
When point is already on that position, then signal an error.
A near full screen is `next-screen-context-lines' less than a full screen.
Negative ARG means scroll downward.
If ARG is the atom `-', scroll downward by nearly full screen.