Add `error` parameters to completion blocks which now pass them. Assert
these errors are always nil in the existing tests.
Flip calls to `selectPendingCount()` so they are async, with a
completion block that asserts about the results instead of asserting
about the return value. Since the closure takes a Result, unwrap it in a
do/catch block at each site; `XCTAssertNoThrow` doesn't help us bubble a
value out from `Result.get()`, and I'd rather not use `try!` here. There
might be a stylistic discussion to be had about this unwrapping, though.
f82be27666