AppendErrors must be able to return a generic type

[SharePointRadi]

The OperationResult class is missing an AppendErrors method that returns a generic type. This existed in 1.x versions, but it is still missing in 2.x and it is causing issues with upgrading to 2.x.

Here is the method.

        /// <summary>
        /// Appends error from <paramref name="otherOperationResult"/> to the current instance.
        /// </summary>
        /// <param name="otherOperationResult">The <see cref="OperationResult"/> to append from.</param>
        /// <returns>The original <see cref="OperationResult"/> with the appended messages from <paramref name="otherOperationResult"/>.</returns>
        public new OperationResult<TResult> AppendErrors(OperationResult otherOperationResult)
        {
            base.AppendErrors(otherOperationResult);
            return this;
        }

Please revert it and add it.

[TonyTroeff]

We need to make sure we have as much backwards compatibility as possible. These methods need appropriate testing in v2.x.
We should add tests ensuring that the AppendErrors method works correctly with both generic and non-generic OperationResult instances. All combinations should be covered, and in every case, the returned instance must retain the type of the target OperationResult (the one receiving the appended error messages).

[SharePointRadi]

Tests should also cover appending errors from one generic type of OperationResult to another generic type. That is the essence of the missing method described by the issue.