DRTVWR-558: Change LLEventDispatcher error action (also LLEventAPI).

Originally the LLEventAPI mechanism was primarily used for VITA testing. In
that case it was okay for the viewer to crash with LL_ERRS if the test script
passed a bad request.

With puppetry, hopefully new LEAP scripts will be written to engage
LLEventAPIs in all sorts of interesting ways. Change error handling from
LL_ERRS to LL_WARNS. Furthermore, if the incoming request contains a "reply"
key, send back an error response to the requester.

Update lleventdispatcher_test.cpp accordingly.

(cherry picked from commit de0539fcbe815ceec2041ecc9981e3adf59f2806)
(cherry picked from commit 4b60941952e97691f11806062f4bc66dd5ac8dae)

indra/llcommon/lleventdispatcher.h

@@ -325,6 +325,7 @@ class LL_COMMON_API LLEventDispatcher
         CLASS* downcast = static_cast<CLASS*>(this);
         add(name, desc, boost::bind(method, downcast, _1), required);
     }
+    void addFail(const std::string& name, const std::string& classname) const;
     std::string try_call_log(const std::string& key, const std::string& name,
                              const LLSD& event) const;
     std::string try_call(const std::string& key, const std::string& name,