Add extra type to errors for override

[mkevinosullivan]

Description

DiscountCode has an errors property that overrides the same property in base.rb, creating a Sorbet type conflict. This commit adds the additional typing to allow the override.

How has this been tested?

Re-running the rest wrapper tests.

Checklist:

• [x] My commit message follow the pattern described in here
• [x] I have performed a self-review of my own code.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have updated the project documentation.
• not applicable ~I have added a changelog line.~