commands: Added test for Response error marshalling

response_test.go

 package commands
 
-import "testing"
+import (
+  "testing"
+  "fmt"
+)
 
 type TestOutput struct {
   Foo, Bar string
@@ -29,4 +32,14 @@ func TestMarshalling(t *testing.T) {
   if output != "{\"Foo\":\"beep\",\"Bar\":\"boop\",\"Baz\":1337}" {
     t.Error("Incorrect JSON output")
   }
+
+  res.SetError(fmt.Errorf("You broke something!"), Client)
+  bytes, err = res.Marshal()
+  if err != nil {
+    t.Error("Should have passed")
+  }
+  output = string(bytes)
+  if output != "{\"Message\":\"You broke something!\",\"Code\":1}" {
+    t.Error("Incorrect JSON output")
+  }
 }