revert over-zealous simplification

I was trying to simplify code flow by removing things with tainted variables. I got too carried away and thought tainted wait handles were the same.
goodcatch
@override-unit-failures

hphp/runtime/ext/asio/continuation_wait_handle.cpp

@@ -72,9 +72,10 @@ Object c_ContinuationWaitHandle::ti_start(const char* cls, CObjRef continuation)
   }
 
   if (UNLIKELY(cont->m_index != -1)) {
-    assert(cont->m_running);
     Object e(SystemLib::AllocInvalidOperationExceptionObject(
-        "Encountered an attempt to start currently running continuation"));
+      cont->m_running
+      ? "Encountered an attempt to start currently running continuation"
+      : "Encountered an attempt to start tainted continuation"));
     throw e;
   }