[Cashtab] Show correct fiat on token creation error msg

Summary: The error msg shown to a user trying to create a token with 0 XEC in their wallet currently is hardcoded to '$' and 'USD', even though the app supports multiple currencies. This diff corrects this oversight by applying the same code used in balance display.

Test Plan:
npm start
Create a new wallet or select a wallet with less than 5.46 XEC
Change currency to anything but USD
Navigate to "Tokens" page and confirm that error msg displays your selected currency, e.g. "You need at least 5.46 XEC (₩ 0.3371 KRW ) to create a token"

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9845