text: Title: Resolving special variables can fail with confusing message
  Body: Considering such example:
  
  ```
  *** Variables ***
  ${TABLE}    example
  
  
  *** Test Cases ***
  Special Variables
      IF    all(item not in $TABLE for item in ("one", "two"))
          Log    ${TABLE}
      ELSE
          Log    not ${TABLE}
      END
  
  ```
  
  I'm not sure if ``all(item not in $TABLE for item in ("one", "two"))`` is supported at all but the error I'm getting looks incorrect - it contains 'internal' naming used in RF (with prefix ``RF_VAR_``):
  
  ```
  NameError: name 'RF_VAR_TABLE' is not defined
  ```