Change error type used for division by zero in assert statements

[alexanderjsummers]

It seems surprising that the following code generates a assert.failed:division.by.zero error rather than not.wellformed:division.by.zero:

method foo() {
        //:: ExpectedOutput(not.wellformed:division.by.zero)
        assert 42 / 0 == 1
}

[mschwerhoff]

For what it's worth: I believe we consistently report well-formedness issues mid-program (and in contrast to pre-/postconditions and invariants) as statement failures. E.g. assert x.f != 0 will fail with assert.failed:insufficient.permission if x.f can't be accessed.