Fix assertions on Result values

[briansmith]

From the Rust coding guidleines:

// Extract the contents of Ok variant; fail if Err
fn assert(self) -> V

// Extract the contents of Err variant; fail if Ok
fn assert_err(self) -> E

The code has been using assert(x.is_ok()) and assert(x.is_err()) instead.

[briansmith]

It turns out that the Rust coding guidelines are out of date. I filed aturon/aturon.github.io#3. Thus, there's nothing to do here.