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)