Instead of returning an error "error doing X", just return "doing X". The
fact that it's returned as an error object says enough about that it's
an error.
This also makes it easier to chain error messages, without seeing the
word "error" in every part of the chain.
This also requires that `TaskID` is no longer a custom type, because that
would cause import cycles. The alternative would be to put the generated
mocks directly into the `worker` package, but I didn't think that was
particularly nice. Maybe this'll be reconsidered later.
