Several users have noticed that they cannot pay satoshis.place or similar places
that have tiny payment amounts if they are not directly connected. This is due
to the forwarding fee dominating the transferred amount.
This commit adds a new option, exempting tiny fees (up to 5 satoshis by default)
from having to pass the maxfeepercent flag. While we could have told users to
tweak maxfeepercent I think it is usefull to have a default exemption.
[Squashed --RR]
These error codes will cause `pay` to retry, so `pay` will never
actually report those error codes.
Those error codes will only get reported at the `sendpay` level.
