Backend/UtxoCoin: stop considering invalid fee estimation results

Reported by Andrew Cann, caught when running with regtest: it could be
dangerous to not look at these values and reject them, given that we use
an average-calculation later. Well, "dangerous", the worst that could
happen is that we use a fee that is too low.

src/GWallet.Backend/UtxoCoin/ElectrumClient.fs

@@ -86,6 +86,11 @@ module ElectrumClient =
     let EstimateFee (numBlocksTarget: int) (stratumServer: Async<StratumClient>): Async<decimal> = async {
         let! stratumClient = stratumServer
         let! estimateFeeResult = stratumClient.BlockchainEstimateFee numBlocksTarget
+        if estimateFeeResult.Result = -1m then
+            return raise <| ServerMisconfiguredException("Fee estimation returned a -1 error code")
+        elif estimateFeeResult.Result <= 0m then
+            return raise <| ServerMisconfiguredException(SPrintF1 "Fee estimation returned an invalid non-positive value %M"
+                                                                  estimateFeeResult.Result)
         return estimateFeeResult.Result
     }