Robot name - Revision 3

Moved the name generation functionality from `RobotNameRegistry` class into `RobotNameGenerator` class as suggested by @bwthomas . Also cleaned up the exception classes. They are less verbose now.

###### Previous PRs

https://github.com/jmmastey/level_up_exercises/pull/187
https://github.com/jmmastey/level_up_exercises/pull/199

I see now why you're using those constants. Still, this just feels awkward. I'm still in favor of _not_ abstracting this particular configuration.