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().
Calling getTrace() on an uninitialized Exception will just return NULL, same as getFile(), getLine(), etc., which seemed OK to me, whereas getTraceAsString() was giving "Invalid argument supplied for foreach()", which seemed not OK. But I can change it if you like.