Do not request empty folders.

Doing this resulted in a 404 on Aditi, and although that was a server
bug; it still doesn't seem to make much sense to do the request in the
first place.
Fix crash in LLTextureFetch::getWorker upon viewer exit.

This is now necessary since the curl thread no longer syncs with the
main thread: it is possible that a request finishes after a texture
fetch thread was shot down but before curl was stopped, and curl
calling BufferedCurlEasyRequest::processOutput while objects that the
responder uses were already destructed (most notably
LLTextureFetch itself).
Add comment with regard to LLSD in the body of pages with an HTTP error code.