On layers.openembedded.org we're seeing requests from some search engine
crawlers requesting the CSV export URL with an invalid branch for the
layer. I couldn't see the referer anywhere in the logs but I suspect it
has to do with some recent cleanup work I did in the database where I
deleted some invalid LayerBranch records - they were probably following
links in a cached version of the webpage. In any event we want to return
404 in this situation rather than an internal server error.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>