Add Error Result type

[paulirwin]

A good use case came up for having an "Error" result type. Although generally recommend that "Exceptions be exceptional" and let them bubble up to the caller, there is a case where we need to handle failures calling an external service from a command handler that fail for some reason, and return that reason to the caller as a possible expected failure (rather than an exceptional Exception).

This should include Type and Message properties. Users should be able to subclass this result type if needed for adding additional data.