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.
Merge branch 'master' of git://github.com/AlericInglewood/SingularityViewer into Cupcake