Suppress compilation errors until there has been a successful restore

- Create a new C# Console app
- Observe C# editor window and Error List

Expected: No errors show up, either in the Error List or as squigglies in the editor
Actual: Errors may be displayed temporarily until the NuGet restore operation completes

Suggested fix: When we open a project, disable errors from the compiler until a restore operation has completed for the project. (We should still probably show the errors if the restore operation completed unsuccessfully, because that might be because one of the dependencies wasn't found, but NuGet will still write a partial assets file with what it was able to restore.)