RPC server code hardening

[icook]

• Add error handling to update_payouts rpc command to allow associating with pre-existing transactions. Solves failure scenario client push associon succ -> fails to record locally -> retry and fail to repeat association, thinking there's a problem when exc is raised by remote
• Add several return values/metrics to return data from all RPC functions to allow for double checks on rpc client side. Paranoid, so very extra (true should mean all actions performed properly in an atomic transaction)