Backend.Tests: add failing test for string error

We noticed in [1] that some servers return error response
that has only a string error instead of an error object,
so we're adding a test to prove the issue.

The error originally happened when I passed uint256
as txId to GetBlockchainTransaction electrum request
when it should've been a string.

[1] #221

src/GWallet.Backend.Tests/Deserialization.fs

@@ -5,6 +5,7 @@ open System
 open NUnit.Framework
 
 open GWallet.Backend
+open GWallet.Backend.UtxoCoin
 
 [<TestFixture>]
 type Deserialization() =

src/GWallet.Backend.Tests/StratumParsing.fs

@@ -41,4 +41,13 @@ type StratumParsing() =
             StratumClient.Deserialize<BlockchainScriptHashGetBalanceResult> errorResponse
             |> ignore<BlockchainScriptHashGetBalanceResult>
         )
-        Assert.That(ex.ErrorCode, Is.EqualTo(-32603))
+        Assert.That(ex.ErrorCode, Is.EqualTo(-32603))
+
+    [<Test>]
+    member __.``can deserialize error result with string error``() =
+        Assert.Throws<ElectrumServerReturningErrorInJsonResponseException> (
+            fun () ->
+                StratumClient.Deserialize<BlockchainTransactionGetResult> """{"error":"bad tx_hash","id":0,"jsonrpc":"2.0"}"""
+                |> ignore
+            )
+        |> ignore