Return or throw error from `plunk.emails.send()`

`plunk.emails.send()` currently returns the following:

```ts
Promise<{  success: true; }>
```

If an issue occurs, this provides no visibility into what the issue. It'd be more useful if 1.) an error object is also returned, or 2.) it throws an error upon failure (requires a breaking version change), so that the app can use a try/catch to catch and log this.