Add a clear error message indicating RPC endpoint fails to connect

[chrisli30]

If we know RPC connection fails, our program should throw an error in log trace clearly indicating that.

Right now the error looks like this

{"level":"info","ts":1721310748.5356233,"caller":"operator/operator.go:148","msg":"ap-avs operator version v1.1.2"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xd8 pc=0xbfde71]

goroutine 1 [running]:
github.com/AvaProtocol/ap-avs/operator.(*Operator).RegisterOperatorWithAvs(0x0)
        /app/operator/registration.go:54 +0x71
github.com/AvaProtocol/ap-avs/operator.RegisterToAVS({0x7ffdb69c6df1?, 0x0?})
        /app/operator/registration.go:23 +0x65
github.com/AvaProtocol/ap-avs/cmd.init.func4(0xc00012ea00?, {0xfddeff?, 0x4?, 0xfddf03?})
        /app/cmd/register.go:16 +0x25
github.com/spf13/cobra.(*Command).execute(0x19c53e0, {0xc000132e00, 0x1, 0x1})
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:987 +0xab1
github.com/spf13/cobra.(*Command).ExecuteC(0x19c59a0)
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
        /go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039
github.com/AvaProtocol/ap-avs/cmd.Execute()
        /app/cmd/root.go:24 +0x1a
main.main()
        /app/main.go:9 +0xf