Debugger Fails to start (or show proper error) when attempting to debug test for inaccessible class

## Environment data
`dotnet --info` output:
.NET Command Line Tools (1.0.4)

Product Information:
Version: 1.0.4
Commit SHA-1 hash: af1e6684fd

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.12
OS Platform: Darwin
RID: osx.10.12-x64
Base Path: /usr/local/share/dotnet/sdk/1.0.4

VS Code version: 1.12.1
C# Extension version: 0.10.0-beta2

## Steps to reproduce

Create non-public test class with `[Fact]` method. Press `debug test`.

## Expected behavior

Understandable error message related to inability to do what's asked.

## Actual behavior

The debugger hangs for a while and then shows "Failed to debug: null".