[ecash-herald] Retry sends on network error failures

Summary: Over days and weeks of operation, ecash-herald will often have some missed msgs due to connectivity issues or an unresponsive API. When we hit this case, we should wait a bit and try again, instead of just failing.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18633