Refactor decodeFileOrFail to have decodeGetFileOrFail, fixing #57.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
This function probably belongs in `Data.Binary.Get` and should probably be called `runGetSomething`. We're going to split off `Data.Binary` from `Data.Binary.Get`, the former being a particular serialization format and the latter being a generic binary data parser.

/cc @dcoutts