Add explicit tests for division by zero to all user-accessible integer

Add explicit tests for division by zero to all user-accessible integer
division and modulo functions, to avoid problems on OS X (which fails to
trap 0 divide at all) and Windows (which traps it in some bizarre
nonstandard fashion).  Standardize on 'division by zero' as the one true
spelling of this error message.  Add regression tests as suggested by
Neil Conway.