[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