`CacheResult` should in theory have either a `file != null` a `future !=
null` or be `disposed`. Apparently this invariant of `CacheResult` is
either being violated somewhere, or the `synchronized` blocks arent as
synchronized as id hoped they were. In fact, `dispose()` does not even
delete the file, so i can really only see this happening because the
first `synchronized` block that writes the file and the second
`synchronized` block that reads from the file hold the same lock.
I have no idea how this would happen, but hopefully this fixes it (since
the dispose didn't have a threading issue reported so far, i feel more
confident leaving the .deleteOnExit in there, but I'm also wrapping any
potential IOExceptions during access, because I am just so confused how
the internal state was broken.