When a Decimal operation raises multiple signals and more than one of
those signals is trapped, the specification determines the order in
which the signals should be handled.  In many cases this order wasn't
being followed, leading to the wrong Python exception being raised.
This commit fixes those cases, and adds extra tests.  The tests are
only enabled when EXTENDEDERRORTESTS is True, since they involve
rerunning each Decimal testcase several times.