Change doc:render-line to fail at end-of-file

Change doc:render-line to fail at end-of-file

Rather than calling the callback with no string, avoid the callback and
return an error when doc:render-line cannot find another line to render.

This has much the same effect for render-lines, and in other cases is
easier to work with.

Signed-off-by: NeilBrown <neil@brown.name>