Add support for skip_temp_err to payViaRoutes

[alexbosworth]

When paying via routes in an MPP context, any error will cause the entire payment to no longer be accessible for new shards

In LND 0.15.1 there was a change in the API surface that allows for attaching new shards if the failure on the route is a temporary channel failure

Potentially this flag should be added to every invocation of the API since it in theory it is always safe, testing is needed to confirm that is in fact the case