Tweak error handling a little bit

- Handle unresolved projects better in build
- Move unresolved package marking to the resolver
- Added ToString to LibraryKey for easy diagnosis of errors

test/Microsoft.DotNet.ProjectModel.Tests/LibraryExporterPackageTests.cs

@@ -50,7 +50,7 @@ namespace Microsoft.DotNet.ProjectModel.Tests
             return new PackageDescription(PackagePath,
                 package ?? new LockFilePackageLibrary(),
                 target ?? new LockFileTargetLibrary(),
-                new List<LibraryRange>(), true);
+                new List<LibraryRange>(), compatible: true, resolved: true);
         }
 
         [Fact]