[fuzz] Test chanmon_consistency payment-send errors are sane

Instead of simply always considering a payment-send failure as
acceptable (and aborting fuzzing), we check that a payment send
failure is from a list of errors that we know we can hit, mostly
around maxing out our channel balance.

Critically, we keep going after hitting an error, as there's no
reason channels should get out of sync even if a send fails.