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

The tasks happening here are at a weird level of abstraction. Lines 11 and 13 are related to saving names, 10 to generating, 9 and 12 to registering them. Consider abstracting differently.