Logging improvements

[rchande]

The OmniSharp logs in dotnet/vscode-csharp#2147 don't make it easy to figure out what's going wrong. In that bug, a user has opened a directory containing a netcoreapp2.0 project that's been restored. For some reason, every file is full of errors that basically indicate that OmniSharp was unable to load reference assemblies.

The logs indicate:

• Omnisharp located a project and successfulled parsed it and determined the TFM
• The /codecheck endpoint find lots of diagnostics due to missing reference assemblies.

By looking at the logs, we aren't able to determine:

• What paths the MSBuild project loader considered for reference assemblies
• What MetadataReferences were added to the OmniSharp workspace

We should add this additional logging to help diagnose issues like dotnet/vscode-csharp#2147

[rchande]

cc @akshita31 @TheRealPiotrP @DustinCampbell