U4-7371 - New Content type editor - Better validation when trying to create a new document/media/member type with the same name as an existing one

Testing notes:

Don't worry about the YSOD when creating a new doc type - this is fixed in a different PR

Test 1

• Create and save a new document type: ct1
• Create another new document type: ct1 - then save, verify that: ** the URL does not redirect ** you get an error beside the alias field: "A content type with this alias already exists" ** you get the standard error message displayed at the bottom ** Then change the name - this will also re-generate an alias and the validation error should be cleared

Test 2

• Same as Test 1, however, don't change the name field, manually change the alias field by unlocking it, verify that: ** the validation error should be cleared

Test 3

• run the above tests for member types and media types

PR: https://github.com/umbraco/Umbraco-CMS/pull/1012

Tested and reviewed (Mads & Claus)

An issue still exists when re-saving an existing doc type

final fix in rev: 1f245ec553e16f1be664d0847264654bc2b1b8f7