Give helpful error message for SDK style projects

Currently if you try to use the EF6 Designer on an SDK style project you will get a Null-Ref exception. This is because SDK-style projects are not supported. However, we should fail earlier and give a more helpful message. IE, show a dialog when the wizard is opened that says "SDK-Style projects are not supported in the EF6 Designer" or something to that effect. Alternatively we could try to not load the Designer options at all when in an SDK context (though I can see that leading to confusion where people think that the Designer is broken/missing as opposed to disabled).