Call $this->__init__() even in native classes

We (WMF) have many classes that look like:

```
<<__NativeData("ZendCompat")>>
class LuaSandboxError extends Exception {}
```

Native::nativeDataInstanceCtor() is mostly a duplicate of
ObjectData::newInstance(), but the Native function inappropriately omits
the \_\_init__() call. So for the test case above, the exception trace is
not initialised.

Fix this, and also give a more informative error message when
Exception::\_\_init__() is not called prior to
Exception::getTraceAsString().
Ah, ok. It seemed like having an uninitialized Exception in any situation was indicative of a bug somewhere. If you know of any situations where that's not the case, leave it, otherwise I think it's best to change it.