Visualizer should try to find winmd for interface and property types,…

[DefaultRyan]

… and should fail more gracefully when not found.

Currently, if the visualizer only tries to load up new winmds for missing runtimeclasses in the case of top-level types and their bases. It then assumes the type graph should be fully resolvable, even for interfaces and properties, and throws an exception if they aren't found at this point.

There are two problems with this approach:

1. Just because a runtimeclass has its winmd loaded, this doesn't guarantee that its interfaces and property types have had their winmds loaded. Like runtimeclass MyApp.Foo { Component.Widget MyProperty; } - we may have loaded MyApp.winmd, but that doesn't mean Component.winmd has been loaded, so we should extend the metadata loading courtesy to interface and property lookups.
2. Following from 1. if we then try to load a missing winmd, and still can't find the type, we shouldn't throw an exception and take down the debugger process. We should fail gracefully and skip those interfaces and/or properties that we couldn't resolve.