test: (low-priority) darkside: RemoveTreeState on non-existent height crashes lightwalletd

[LarryRuane]

This is a minor bug but should be fixed. To reproduce:

$ go run main.go --no-tls-very-insecure --log-file /dev/stdout --darkside-timeout 999999 --darkside-very-insecure

in another window:

$ grpcurl -plaintext -d '{"saplingActivation": 663150,"branchID": "bad", "chainName":"x"}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/Reset
$ grpcurl -plaintext -d '{"height": 663150}' localhost:9067 cash.z.wallet.sdk.rpc.DarksideStreamer/RemoveTreeState

It's required to add a tree state at this height first, but not doing so shouldn't crash the server; it should generate a reasonable error message and continue running.