App crashes when there is not external activities that matches intent

Avoid crashing when calling external activities and the phone doesn't have any matching activity.

### Expected

Show a Toast that the phone doesn't have a matching activity.

### Observed

Crashes. For example, this crash when exporting notes:

```
No Activity found to handle Intent { act=android.intent.action.CREATE_DOCUMENT cat=[android.intent.category.OPENABLE] typ=application/json (has extras) }
```

### Reproduced
**Note:** the device doesn't have to have a file manager app.

1. Go to Settings
2. Tap on Export Notes
3. See the app crashes

Device Make: Motorola
Device Model: Moto G 2014
Android Version: 7.1.2
Simplenote Version: 2.19

Start from Settings > Export Notes and trace the external activity launch that handles the CREATE_DOCUMENT intent. Reproduce on a device without a matching file manager, then verify that exporting notes shows a Toast explaining that no matching activity is available instead of crashing.

Written by the indexing model from the issue text.