Return 404 for absent PDF, not 500

[bensteinberg]

For cases that have no PDF file, like this one from Fastcase, an attempt to load a PDF file produces an error:

TypeError at /pdf/12609670/BNSF Ry. Co. v. Tyrrell.pdf
unsupported operand type(s) for +: 'NoneType' and 'str'

It should probably produce a 404 instead.