Automatically render a view if command object has errors

It is based on discussion with JeffScottBrown on this thread
http://stackoverflow.com/questions/24742294/refactoring-grails-command-validation

Basically, most of the actions which accepts command objects do some thing like this

``` groovy
def add(CreatePersonCommand command) {
if(command.hasErrors()) {
redirect(action: 'add')
return
}
}
```

It would be great if there was a built in support in grails to render a view if command object has errors
Some thing like this

``` groovy
@Validate(view:"someView")
def add(CreatePersonCommand command) {

}
```

Start with the `add(CreatePersonCommand command)` action shown in the issue and the proposed `@Validate(view:"someView")` entry point. Trace how Grails actions handle command-object validation errors, then define and test the automatic view-rendering behavior for invalid commands.

Written by the indexing model from the issue text.