Better handling of errors when submitting TXs via NtN

[agaffney]

We currently panic on error.

{"level":"info","timestamp":"2024-07-08T06:20:42Z","caller":"miner/worker.go:80","msg":"starting 2 workers"}
{"level":"info","timestamp":"2024-07-08T06:20:42Z","caller":"miner/worker.go:64","msg":"stopped workers"}
panic: async: muxer error: read tcp <ip>:48842-><ip>:3001: read: connection reset by peer

goroutine 3565 [running]:
github.com/blinklabs-io/bluefin/internal/tx.submitTxNtN.func1()
        /home/ubuntu/bluefin/internal/tx/tx.go:442 +0x8e
created by github.com/blinklabs-io/bluefin/internal/tx.submitTxNtN in goroutine 3551
        /home/ubuntu/bluefin/internal/tx/tx.go:439 +0x1a5