Handle Runtime error when connecting to invalid rippled server

[pkcs8]

Runtime error should be handled gracefully.

$ ./bin/platform-server
2023/12/19 11:21:46 Loading configuration file: .env
2023/12/19 11:21:46 WS connection error: ws://localhost:6006 dial tcp 127.0.0.1:6006: connect: connection refused
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x94522a]

goroutine 1 [running]:
github.com/gorilla/websocket.(*Conn).WriteMessage(0x18?, 0xc0001708e8?, {0xc0001708e8, 0x13, 0x18})
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