Fix unhandled exception when target is unavailable (#469)

* wip

* fix

* 31

* known

src/WireMock.Net/Owin/WireMockMiddleware.cs

@@ -129,7 +129,7 @@ namespace WireMock.Owin
             catch (Exception ex)
             {
                 _options.Logger.Error($"Providing a Response for Mapping '{result?.Mapping?.Guid}' failed. HttpStatusCode set to 500. Exception: {ex}");
-                response = ResponseMessageBuilder.Create(JsonConvert.SerializeObject(ex), 500);
+                response = ResponseMessageBuilder.Create(ex.Message, 500);
             }
             finally
             {

src/WireMock.Net/Serialization/LogEntryMapper.cs

@@ -2,9 +2,8 @@
 using WireMock.Admin.Mappings;
 using WireMock.Admin.Requests;
 using WireMock.Logging;
-using WireMock.Types;
 using WireMock.ResponseBuilders;
-using WireMock.Util;
+using WireMock.Types;
 
 namespace WireMock.Serialization
 {