[chore] Change order of error checking after PostInbox (#3394)

Check for malformed errors embedded inside error *first*, then check for gtserror.WithCode.

internal/federation/federatingactor.go

@@ -170,12 +170,6 @@ func (f *federatingActor) PostInboxScheme(ctx context.Context, w http.ResponseWr
 	//
 	// Post the activity to the Actor's inbox and trigger side effects.
 	if err := f.sideEffectActor.PostInbox(ctx, inboxID, activity); err != nil {
-		// Check if a function in the federatingDB
-		// has returned an explicit errWithCode for us.
-		if errWithCode, ok := err.(gtserror.WithCode); ok {
-			return false, errWithCode
-		}
-
 		// Check if it's a bad request because the
 		// object or target props weren't populated,
 		// or we failed parsing activity details.
@@ -193,6 +187,12 @@ func (f *federatingActor) PostInboxScheme(ctx context.Context, w http.ResponseWr
 			return false, gtserror.NewErrorBadRequest(errors.New(text), text)
 		}
 
+		// Check if a function in the federatingDB
+		// has returned an explicit errWithCode for us.
+		if errWithCode, ok := err.(gtserror.WithCode); ok {
+			return false, errWithCode
+		}
+
 		// Default: there's been some real error.
 		err := gtserror.Newf("error calling sideEffectActor.PostInbox: %w", err)
 		return false, gtserror.NewErrorInternalError(err)