Add semi-functional exception throwing.

We still need a way to call <init> on the new exception instance, in particular.

Change-Id: I839365d20288208cb8847253380cf654a35aeddd
diff --git a/src/thread.h b/src/thread.h
index e065f68..8bde370 100644
--- a/src/thread.h
+++ b/src/thread.h
@@ -147,6 +147,9 @@
     exception_ = new_exception;  // TODO
   }
 
+  void ThrowNewException(const char* exception_class_name, const char* fmt, ...)
+      __attribute__ ((format(printf, 3, 4)));
+
   void ClearException() {
     exception_ = NULL;
   }