This fixes a slow test on this branch, which appears to be caused by an
interaction between using `postMessage` to serialize native Error
objects and the `source-map-support` Karma plugin that we use.

Sending native Errors via `PortRPC` currently doesn't work in all
browsers so I expect we'll want to implement our own serialization of
errors in `PortRPC` to work around that. That can also work around the
`source-map-support`/`postMessage` issue in Chrome in future.

To move the Browserify => Rollup conversion forwards, this commit just
changes the test to use an Error-like rather than actual Error object.