text: Title: Templates should be explicitly prohibited with WHILE
  Body: We support templates with IF and FOR but not with TRY or WHILE. With TRY templates are explicitly prohibited, but with WHILE they are not. For example, the following is executed 10k times (default iteration limit) and fails each time with a strange error "Keyword name cannot be empty.".
  
  ```robotframework
  *** Test Cases ***
  Example
      [Template]    Log
      WHILE    True
          Hello!
      END
  ```
  
  This should be changed so that execution fails immediately with a clear message like "WHILE does not support templates.".
  
  With TRY the validation that templates aren't used is currently done in the phase when the parsing model is transformed to the execution model. It would be better to do that already earlier when the parsing model is validated to make the error visible also for IDE plugins and other tools working with the parsing model. This change can be done as part of this issue at the same time when validation is added to WHILE.