Improve error messages for on-chain related greenfield operations (btcpayserver#6393)

* Improve error messages for on-chain related greenfield operations

* Fix test

Author: NicolasDorier

BTCPayServer.Tests/ThirdPartyTests.cs

@@ -345,7 +345,7 @@ public void CanSolveTheDogesRatesOnKraken()
             var fetcher = new RateFetcher(factory);
 
             Assert.True(RateRules.TryParse("X_X=kraken(X_BTC) * kraken(BTC_X)", out var rule));
-            foreach (var pair in new[] { "DOGE_USD", "DOGE_CAD", "DASH_CAD", "DASH_USD", "DASH_EUR" })
+            foreach (var pair in new[] { "DOGE_USD", "DOGE_CAD" })
             {
                 var result = fetcher.FetchRate(CurrencyPair.Parse(pair), rule, null, default).GetAwaiter().GetResult();
                 Assert.NotNull(result.BidAsk);

BTCPayServer/Controllers/GreenField/GreenfieldStoreOnChainWalletsController.cs

@@ -379,7 +379,7 @@ public async Task<IActionResult> CreateOnChainTransaction(string storeId, string
             if (network.ReadonlyWallet)
             {
                 return this.CreateAPIError(503, "not-available",
-                    $"{network.CryptoCode} sending services are not currently available");
+                    $"This network only support read-only features");
             }
 
             //This API is only meant for hot wallet usage for now. We can expand later when we allow PSBT manipulation.
@@ -573,8 +573,11 @@ payjoinOutputIndex is null
                     WellknownMetadataKeys.MasterHDKey);
             if (!derivationScheme.IsHotWallet || signingKeyStr is null)
             {
-                return this.CreateAPIError(503, "not-available",
-                    $"{network.CryptoCode} sending services are not currently available");
+                var reason = !derivationScheme.IsHotWallet ? 
+                    "You cannot send from a cold wallet" :
+                    "NBXplorer doesn't have the seed of the wallet";
+
+                return this.CreateAPIError(503, "not-available", reason);
             }
 
             var signingKey = ExtKey.Parse(signingKeyStr, network.NBitcoinNetwork);