Dynamically scaffolded Edit-View gives incorrect feedback for illegal Updates

### Steps to Reproduce
1. create an exampleDomain with a string that is unique
2. create a controller for that domain with static scaffold = exampleDomain
3. start the application
4. create two objects ("1" & "2")
5. edit the string in object "1" to "2"
### Expected Behaviour

The application should give an error message that the string is unique and thus can't be renamed
### Actual Behaviour

The application responds that the update was successful while the update did not actually take place
### Environment Information
- **Operating System**: Ubuntu
- **Grails Version:** 3.1.6
- **JDK Version:** 1.7
### Example Application

See [attachment](https://github.com/grails/grails-core/files/270309/UpdateErrorExample.zip)

Start with the attached UpdateErrorExample.zip and reproduce the dynamically scaffolded Edit-View update on Grails 3.1.6 using the listed steps. Investigate why renaming object "1" to the duplicate string "2" reports success, and consider the issue done when the application shows the uniqueness error instead.

Written by the indexing model from the issue text.