Fixes VSTS Bug 935147: [FATAL] System.InvalidOperationException exception in Mono.Addins.ExtensionNode.get_Addin()
Fixes VSTS Bug 935147: [FATAL] System.InvalidOperationException exception in Mono.Addins.ExtensionNode.get_Addin()

https://devdiv.visualstudio.com/DevDiv/_workitems/edit/935147

node.AddIn crashed and caused a fatal exception. That's now fixed and the error is reported.

mono/mono-addins,mono/mono-addins