Extend message string to support a list of messages or exceptions.

[olivegamestudio]

Context

At present only one error can be set per result, I am thinking that we may need to consider a series of exceptions or callstack and to capture this.

/// <summary>
/// Gets the error message associated with the result.
/// </summary>
public string Error { get; private set; }

/// <summary>
/// Gets the error code associated with the result.
/// </summary>
public int ErrorCode { get; private set; }