[electrum] wrap the error message when exchange rate API fails

Summary:
Provide a better user experience by not showing a full python error stack for common URL or JSON errors.
ref T3816

Test Plan:
Try loading invoices with various errors (Tools > Invoice > Load and pay invoice).
With the example provided in T3816, replacing "api.binance.com" with "apisdqdsfgsqdg.binance.com" causes a JSONError.
Replacing "symbol=XECUSDT" with "symbol=XECUSGT" causes a HTTPError.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T3816

Differential Revision: https://reviews.bitcoinabc.org/D18052