Print a warning if FindAssembliesWithReferencesTo cannot find file

[pranavkm]

Missing assemblies will fail the build, but since FindAssemblyWithReferencesTo gets first dibs, it throws the exception rather than csc. We should instead consider printing a warning and letting later parts in the build fail.

[pranavkm]

We went with using LogFromException, so the outcome looks like what csc prints when it cannot find an assembly (instead of a stack trace). Much better.