Nanoscope on emulator with multiple devices

Can we do one of the following?
- support passing `-s DEVICE_ID` to target nanoscope command at one of many open emulators
- improve the error message for running `nanoscope emulator` when there are multiple emulators open? Currently it just says:

> The OS running on your device is not supported. In order to install the Nanoscope ROM, run the following...