Fix incorrect error message implementation in WordPressAPIError

[crazytonyli]

Description

I made two mistakes in the existing implementation:

1. A copy-paste error where I should have made WordPressAPIError conforms to LocalizedError.
2. The implementation should always return a non-nil error message, but I missed that in the endpointError case, because errorDescription is String?.

Testing Details

I have added unit tests in this PR to verify the fixes.

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[mokagio]

Would it be worth adding tests for the other errorDescription mappings, or since they are logic free we can do without?

I don't have an answer for that, but the criteria I usually use is "how likely is this to change and how important is it for us that it changes intentionally"

[mokagio]

The implementation should always return a non-nil error message

Gotcha. So even though errorDescription at the LocalizedError level allows for a nil value, we agree not to do that.

Would it be worth documenting it?

[mokagio]

Maybe... there could be an internal _errorDescription or strongErrorDescription or some other better name, that is String and public var errorDescription: String? could delegate to that. The idea being to have the compiler help us remember not to return any Optional.none.

[crazytonyli]

Good point. I didn't make more changes to the implementation because I thought the returning nil case is covered by the added unit tests. But I'll add a bit documentation too.