Trezor Wallet check if the service is online

[rdig]

Since we are relying solely on Trezor's servers to operate the Trezor Hardware, it's a good idea to make sure that the service is up and running before attempting to create the new TrezorWallet instance.

This could be done by making a OPTIONS fetch() request to the Trezor service and checking if the reponse status is 200.