Remove stub mutant task
Add underscore to unused local variable
Change exception to handle formatting the error message
Remove unnecessary return value from method
Change constant into class variable

* The object this references isn't exactly a constant until
  finalized, and the object doesn't need to be part of the public
  API so a class variable seems like a better fit.
Fix code formatting
Fix spec to test more precise error type
Extract dsl methods from emitter base class into module
Fix reek threshold