[electrum] fix error when the exchange rate API is down

Summary: fixes https://github.com/Bitcoin-ABC/bitcoin-abc/issues/576

Test Plan: Run electrum-abc, menu Tools > Invoice > Create new invoice, type an amount and see the conversion occur in the label next to the widget

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18783