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

This is probably unnecessary and actually detrimental. If I provide an object that implements the `#add` method to your satisfaction, why do you care that it's a `RobotNameRegistry`?