DirectoryAssemblyResolver usage needs better error reporting

[jonpryor]

Context: https://developercommunity.visualstudio.com/content/problem/414579/error-10.html

Remember commit 38c365b / PR #1551 / Issue #1532, in which attempts to find an assembly which do not exist results in terrible error messages, a'la

System.IO.FileNotFoundException: Could not load assembly 'Example'. Perhaps it doesn't exist in the Mono for Android profile?

The same "bad error message" scenario exists in <ResolveLibraryProjectImports/> as well.

All locations which "resolve assemblies" -- such as DirectoryAssemblyResolver -- should be reviewed to ensure that useful error messages are produced. A FileNotFoundException is not usable.

VS bug #801123

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