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

Summary: 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().
Closes #3088
Created from: https://github.com/facebook/hhvm/pull/3088

Reviewed By: @swtaarrs

Differential Revision: D1435376