Display a warning in the Image Styles Administration page when a style with the same Identifier already exists

Steps to reproduce

1. Go to Administer Wiki > Look & Feel > Image Styles
2. Create a new Image Style (e.g. with Identifier: 'Style1')
3. In order to create another Image Style, in the 'New Image Style' field, input the same Identifier, e.g. 'Style1'
4. Click 'Create the image style' button

Expected results

A small red warning below the input field or near is displayed, informing the user that the respective Identifier already exists.

Actual results

The user is directed to ImageStyleClass page (http://localhost:8080/xwiki/bin/view/Image/Style/Code/ImageStyleClass) where there is a warning message stating that 'The target page already exists.'.

This can be confusing for the users since the title of the page and the message imply that the user should have technical knowledge about the Image Style implementation and that he landed on the Image Style class page.