swapd panics on startup if RPC server port is in use
as title says, if the RPC port is in use the node panics on startup. it should not panic but fail with a nice "port in use, pick another port with --rpc-port" message.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1313f11]

goroutine 1 [running]:
github.com/athanorlabs/atomic-swap/rpc.(*Server).Start(0x0)
        /go/pkg/mod/github.com/athanorlabs/atomic-swap@v0.1.1/rpc/server.go:169 +0x31
github.com/athanorlabs/atomic-swap/daemon.RunSwapDaemon({0x1c26bf8, 0xc0000c5720}, 0xc0000c4a50)
        /go/pkg/mod/github.com/athanorlabs/atomic-swap@v0.1.1/daemon/swap_daemon.go:162 +0xca7
main.runDaemon(0xc0001e00c0)
        /go/pkg/mod/github.com/athanorlabs/atomic-swap@v0.1.1/cmd/swapd/main.go:287 +0x3a8
github.com/urfave/cli/v2.(*Command).Run(0xc0000e58c0, 0xc0001e00c0, {0xc0000402c0, 0x1, 0x1})
        /go/pkg/mod/github.com/urfave/cli/v2@v2.25.1/command.go:274 +0x9eb
github.com/urfave/cli/v2.(*App).RunContext(0xc0000225a0, {0x1c26bf8?, 0xc0000c5720}, {0xc0000402c0, 0x1, 0x1})
        /go/pkg/mod/github.com/urfave/cli/v2@v2.25.1/app.go:332 +0x616
main.main()
        /go/pkg/mod/github.com/athanorlabs/atomic-swap@v0.1.1/cmd/swapd/main.go:231 +0x117
```