fix error placement to be a bit less horrid

move to above field rather than next to
improve the display of the screen number list

highlight current 'screen' in a more obvious manner and also
push away from the top to stop it hitting the mysoc logo
buttonise the prev/next links

makes them a bit more obvious by adding the button mixin
increase minimum height of 'screens'

prevent jumping about of buttons for all but the two longest questions