Fix handling of Singular kernel errors in libsingular

[fingolfin]

The way we handle kernel errors in our C++ wrapper code is wrong. As a result, all kinds of things can break. Part of the problem is this:

static void WerrorS_and_reset(const char * s)
{
    errorreported = 0;
...

We reset the error flag in our Singular kernel error callback. But that can lead to the Singular kernel next executing things it normally wouldn't have executed, because it is no longer aware that something just went wrong!

So we must reset this flag elsewhere, later.

One strategy would be to just reset it at the end of every wrapper function.... Not nice but I can't think of anything much better right now :-(