Change validateDefinition and validateSubmission to throw error and remove Results object

Change these functions:

```js
PresentationExchange.validateDefinition({ presentationDefinition })
PresentationExchange.validateSubmission({ presentationSubmission })
```

to throw an exception rather than return a results object if there is a problem with the submission or definition.

Optionally can refactor to:
```js
PresentationExchange.validate(PresentationDefinition | PresentationSubmission)
```

to make the api surface area even smaller