Automatically retry fetch whenever polling transactions or accounts failed

Right now, I've set this to 3 times, to see if it helps with my own setup. Maybe it should be configurable by the user.

This is an attempt to counter spurious networking issues I'm encountering with the Crédit Mutuel bank, which randomly disconnects me only from my server machine. We'd probably need less stupid checks to decide whether we retry or not (e.g. only retry when there's a network error, or some error among a known set of errors).