Add out variance to ApiResult.Failure generic type

[serge-slack]

Summary

Add out variance to ApiResult.Failure<E> to make subtypes implementing Failure<Nothing> such as NetworkFailure conform to Failure<*>

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @serge-slack to sign the Salesforce Inc. Contributor License Agreement.