Report serialization errors in conformance tests

csharp/src/Google.Protobuf.Conformance/Program.cs

@@ -105,6 +105,8 @@ namespace Google.Protobuf.Conformance
             {
                 return new ConformanceResponse { ParseError = e.Message };
             }
+            try
+            {
                 switch (request.RequestedOutputFormat)
                 {
                     case global::Conformance.WireFormat.JSON:
@@ -116,6 +118,11 @@ namespace Google.Protobuf.Conformance
                         throw new Exception("Unsupported request output format: " + request.PayloadCase);
                 }
             }
+            catch (InvalidOperationException e)
+            {
+                return new ConformanceResponse { SerializeError = e.Message };
+            }
+        }
 
         private static int? ReadInt32(BinaryReader input)
         {