Confusing error message for non-parallelizable plugin

Babel accepts plugins as (among other things) either a string denoting a module to require, or a tuple containing that string and optionally some configuration for that plugin. It will also accept a singleton array with just the module path as a sort of degenerate version of the second case, i.e.

```js
plugins: [
'/path/to/some/plugin.js',
['/path/to/some/plugin.js', { config: true }],
['/path/to/some/plugin.js']
]
```

For the third entry, the parallelization logic interprets that plugin as `['/path/to/some/plugin.js', undefined]`, and since `undefined` isn't JSON-serializable, the plugin is considered unparallelizable. That's [easy enough to work around](https://github.com/typed-ember/ember-cli-typescript/pull/351), but the error message we got with `throwUnlessParallelizable` turned on looked like:

```
[broccoli-persistent-filter:Babel > [Babel: ember-data]: Babel: ember-data] was configured to `throwUnlessParallelizable` and was unable to parallelize a plugin.
plugins:
1: name: unknown, location: unknown
```

If the relevant plugin path had been included in that output, it would have helped with tracking down the source of the problem.