provide way to handle M_CONSENT_NOT_GIVEN

**Describe the bug**

When registering a new guest user via matrix-js-sdk, I get an error `M_CONSENT_NOT_GIVEN` asking me to accept the term of Matrix. Unfortunately, I can't find any way to get this URL directly in `matrix-js-sdk` (for instance to display this form to the user to sign).

**To Reproduce**
Load this script after putting the SDK in `lib/browser-matrix.min.js`.

```

Merlin-Peres Magic Square








class MatrixClient {
constructor(matrix_instance="https://matrix.org", room_id="!ThqzNEDFDpeWJYEObP:matrix.org") {
this.matrix_instance = matrix_instance
this.room_id = room_id
}

async start() {
const tmpClient = window.matrixcs.createClient(this.matrix_instance);
const { user_id, device_id, access_token } = await tmpClient.registerGuest()
console.log("access_token: ",access_token)
this.client = window.matrixcs.createClient({ baseUrl: this.matrix_instance,
accessToken: access_token,
userId: user_id,
deviceId: device_id})
this.client.setGuest(true);
await this.client.startClient({initialSyncLimit: 10})
console.log("Client started!")
this.client.on("sync", (state, prevState, result) => {
console.log("On sync: State %s -> %s", prevState, state)
console.log("On sync: Result: %j", result)
this.client.joinRoom(this.room_id).then(function() {
console.log("Joined %s", roomId);
});
})
}
}
matrixClient = new MatrixClient()
matrixClient.start()

```

**Expected behavior**

I expect to have a way to handle `M_CONSENT_NOT_GIVEN`.

Start at the registerGuest() call in the provided browser script and trace how the SDK exposes registration errors, especially M_CONSENT_NOT_GIVEN. Check the browser bundle entry point referenced as lib/browser-matrix.min.js and identify the consent URL or handling information available there. Done means a guest registration flow can obtain and handle the consent requirement for the user.

Written by the indexing model from the issue text.