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

That said, `DEFAULT_SCHEMA` is a weird name for what is essentially just a format. You can also turn it into a regex by just doing `/[[:upper:]]etc etc/`, which seems more reasonable.