[Cashtab] Handle unknown tokens

Summary: In-node chronik may return a tokenId of 000000000000.... for an unknown token. Handle this case, as an API call to chronik.token(000...) will always throw an error.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15729