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 a weird construction. Consider using `.tap` maybe?

```
generator.call.tap do |name|
  validate(name)
end
```