Better error handling for invalid books and places

[nrabinowitz]

Right now, the application just hangs if you try to go to an invalid book or place URL (e.g. http://gap.alexandriaarchive.org/gapvis/index.html#book/spam). In these cases, it would probably be best to redirect to the index (for invalid books) or the book summary (for invalid pages), unsetting the relevant state variable.