Custom content providers returning incorrect data cause hard to understand exception

Description

A custom content provider can cause a KeyNotFoundException when content is returned that was not expected.

A better exception should be thrown, such as:
"Content provider X returned unexpected content link Y that was not in the requested list of content links"

[KeyNotFoundException: The given key was not present in the dictionary.]
System.Collections.Generic.Dictionary`2.get_Item(TKey key) +12347693
EPiServer.Core.ContentProvider.GetScatteredContents(IEnumerable`1 contentLinks, ILanguageSelector selector) +784
EPiServer.Core.ContentProvider.LoadBatched(IList`1 contentReferences, ILanguageSelector selector) +40
EPiServer.Core.ProviderPipelineImplementation.GetItems(ContentProvider provider, IList`1 contentLinks, LoaderOptions loaderOptions) +225
EPiServer.Core.DefaultContentLoader.GetChildren(ContentReference contentLink, LoaderOptions loaderOptions, Int32 startIndex, Int32 maxRows) +1668
EPiServer.Core.DefaultContentLoader.GetChildren(ContentReference contentLink, LoaderOptions loaderOptions) +67
EPiServer.Core.DefaultContentRepository.GetChildren(ContentReference contentLink, LoaderOptions loaderOptions) +66