Revert "Fix error unmarshaling"

This reverts commit 982b2c7d.

integrationtest/errors_test.go

-//go:build integration
 // +build integration
 
 package integrationtest
@@ -131,7 +130,7 @@ func TestIntegration_Error_Validation(t *testing.T) {
 		t.Fatalf("CreateUser() did not return a structured error: %v", err)
 	}
 
-	var fields map[string]string
+	var fields map[string][]string
 	if err := json.Unmarshal(httpErr.data, &fields); err != nil {
 		t.Fatalf("error unmarshaling JSON error: %v", err)
 	}