Backend: handle null response from Eth server

Introduce WeirdNullResponseException type and raise it when
response for balance request for Ethereum returns json with
"result" field value of `null`. This way server will be marked
as faulty instead of crashing the application.

Fixes #282

src/GWallet.Backend/Ether/EtherExceptions.fs

@@ -95,3 +95,7 @@ type UnhandledWebException =
         }
     new (info: SerializationInfo, context: StreamingContext) =
         { inherit Exception (info, context) }
+
+/// Exception indicating that response JSON contains null value where it should not
+type WeirdNullResponseException(message: string) =
+    inherit CommunicationUnsuccessfulException(message)

src/GWallet.Backend/Ether/EtherServer.fs

@@ -561,7 +561,7 @@ module Server =
                                 return! Async.AwaitTask task
                             }
                     if Object.ReferenceEquals(balance, null) then
-                        failwith "Weird null response from balance job"
+                        raise <| WeirdNullResponseException "Weird null response from balance job"
                     return UnitConversion.Convert.FromWei(balance.Value, UnitConversion.EthUnit.Ether)
                 }
                 GetRandomizedFuncs currency web3Func

src/GWallet.Backend/ServerManager.fs

@@ -128,6 +128,8 @@ module ServerManager =
                 let web3Func (web3: Ether.SomeWeb3): Async<decimal> =
                     async {
                         let! balance = Async.AwaitTask (web3.Eth.GetBalance.SendRequestAsync ETH_GENESISBLOCK_ADDRESS)
+                        if isNull balance then
+                            raise <| Ether.WeirdNullResponseException "Weird null response from balance job"
                         return balance.Value |> decimal
                     }