[ecash-herald] Make API failure logs less noisy

Summary:
This happens every once in awhile and so far it has always been related to coingecko rate limit or coingecko downtime. Stop dumping the whole error.

Note: this is related to a series of improvements in ecash-herald following a recent ecash-herald failure, where numerous potential issues appeared in the logs and the real problem was difficult to isolate. We do not really care about (routine) coingecko failures, which the app handles anyway, so make them less noisy.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18631