Cleanup a bit of exception/error handling between the VM and the debugger

We had two similar-but-different functions for getting a notification from the VM about an exception. Cleaned that up by using the proper one for a thrown exception where appropriate, and moving the old one into a hook (like the other VM->debugger hooks) specifically for error messages.

hphp/runtime/debugger/cmd/cmd_interrupt.cpp

@@ -43,7 +43,7 @@ void CmdInterrupt::sendImpl(DebuggerThriftBuffer &thrift) {
     thrift.write(m_site->getNamespace());
     thrift.write(m_site->getClass());
     thrift.write(m_site->getFunction());
-    Variant e = m_site->getException();
+    Variant e = m_site->getError();
     if (e.isNull()) {
       thrift.write("");
     } else if (e.isObject()) {