Handle server side errors

- [x] add login field
- [x] extend `web.coffee` server to raise validation errors
- [ ] catch 422 errors in the controller
- [ ] display flash inside the dialog
- [ ] highlight the errored field

### Sample error from the grails app

```json
{ "code" : 400,
  "errors" : { "user" : { "login" : "Property [login] of class [class grinder.User] with value [login-1] must be unique" } },
  "message" : "User save failed",
  "messageCode" : "default.not.saved.message",
  "status" : "error"
}
```