Change from raising error to show an alert if instance exists

[Alfonso Martínez de Lizarrondo]

Split from #8226

I think that many new users don't realize that they have errors in the console (after all IE is the only browser that currently shows clearly when a page has a js error). We must remember that many people aren't javascript developers, that's why they use things like the asp.net or Java adapters, they just know how to work at the server side and when they try to copy some javascript code as shown in some examples and doesn't work they don't understand what's the problem.

So I strongly suggest to change throw '[CKEDITOR.editor] The instance "' + previous.name + '" already exists.'; to alert('[CKEDITOR.editor] The instance "' + previous.name + '" already exists.'); }