GetReleaseNotes will throw an exception when blank, empty, or null

### Per conversation on twitter: 

"One thing that really needs work is, Shimmer doesn't throw useful errors when devs don't do things right (i.e. if you have a missing entry in NuSpec) - would be great to get some friendly errors"

@xpaulbettsx: Is this the kind of thing your looking for? Exceptions if entries in the NuSpec file aren't filled in, or where you thinking something with checking to make sure the dependencies exist in the packages folder?