fix: error parsing

[Justintime50]

Description

• Fixes error parsing
  • Allows for alternative format of errors field
  • Corrects available properties of an EasyPostError and ApiError (code and field removed from EasyPostError, message unfurled and explicitly added to ApiError)
  • Removes unused Error model

Testing

• Adds real (missing from this lib for whatever reason) error tests that are the same as other libs
• Adds alternative error format test
• Adds address verification error test

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)