An error is now issued for `in`/`out` contracts of `nothrow` functions that may throw

This used to issue a deprecation, it is now an error:
```
void test() nothrow
in
{
    throw new Exception(null); // Error: `in` contract may throw but function is marked as `nothrow`
}
out
{
    throw new Exception(null); // Error: `out` contract may throw but function is marked as `nothrow`
}
do
{
}
```