Throw an exception if ambiguous images are found during packaging

[danbelcher-MSFT]

Currently, when retrieving a specific image for a language's listing, StoreBroker will throw a warning if multiple images are found with the same name under the language's image folder. StoreBroker then picks the first image it found.

Instead, StoreBroker should throw an exception and not attempt to resolve the ambiguity. We should handle this as a breaking change and increment the major version of the NuGet package.