When dealing with proc macros, there are two very different kinds of
errors:
* first, usual errors of "proc macro panicked on this particular input"
* second, the proc macro server might day if the user, eg, kills it
First kind of errors are expected and are a normal output, while the
second kind are genuine IO-errors.
For this reason, we use a curious nested result here: `Result<Result<T,
E1>, E2>` pattern, which is 100% inspired by http://sled.rs/errors.html
