if we have to use the fallback fee rate, log the error that caused us to do so
if we have to use the fallback fee rate, log the error that caused us to do so

Summary: this would have helped us debug a client issue recently

Reviewers: alex, arik, benchan

Reviewed By: benchan

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D5142

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS