Handle pdf-reader gem errors

The pdf-reader gem will not handle every single PDF. Here we rescue the two common errors caused by strange PDFs and carry on. There is little point reporting these errors to our exception tracking as there isn't much that we can do about malformed or unsupported PDFs.

See https://errbit.production.alphagov.co.uk/apps/53020d6c0da11585f10000e7/problems/538c79ec0da11537fa001282?notice=2