When VFS4G Mount process can't download sizes, it dies

[2019-09-03 10:49:45.0889 -07:00] Error {"Area":"GitIndexProjection","Exception":"GVFS.Virtualization.Projection.SizesUnavailableException: Failed to download size for F42607E576DF84999180AC94C7B196C73A7F7B54\n   at GVFS.Virtualization.Projection.GitIndexProjection.FolderData.PopulateSizesFromRemote(ITracer tracer, GVFSGitObjects gitObjects, BlobSizesConnection blobSizesConnection, HashSet`1 missingShas, List`1 childrenMissingSizes, CancellationToken cancellationToken) in /Users/mobileci/jenkins/workspace/gvfs-build-git-vfs4git/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.FolderData.cs:line 150\n   at GVFS.Virtualization.Projection.GitIndexProjection.FolderData.PopulateSizes(ITracer tracer, GVFSGitObjects gitObjects, BlobSizesConnection blobSizesConnection, Dictionary`2 availableSizes, CancellationToken cancellationToken) in /Users/mobileci/jenkins/workspace/gvfs-build-git-vfs4git/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.FolderData.cs:line 67\n   at GVFS.Virtualization.Projection.GitIndexProjection.ReExpandFolder(BlobSizesConnection blobSizesConnection, String relativeFolderPath, HashSet`1 existingPlaceholders) in /Users/mobileci/jenkins/workspace/gvfs-build-git-vfs4git/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.cs:line 1385\n   at GVFS.Virtualization.Projection.GitIndexProjection.UpdatePlaceholders() in /Users/mobileci/jenkins/workspace/gvfs-build-git-vfs4git/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.cs:line 1184\n   at GVFS.Virtualization.Projection.GitIndexProjection.ParseIndexThreadMain() in /Users/mobileci/jenkins/workspace/gvfs-build-git-vfs4git/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.cs:line 1037","ErrorMessage":"ParseIndexThreadMain caught unhandled exception, exiting process"}

Currently, failure to download sizes results in the mount process crashing and then failing to start (for the same reason).

Perhaps we can find a more graceful solution? Perhaps notifying the user of the problem via a notification and exiting?

Start with GVFS/GVFS.Virtualization/Projection/GitIndexProjection.FolderData.cs, especially PopulateSizesFromRemote and PopulateSizes, then follow the callers in GitIndexProjection.cs. Reproduce the missing-size failure and determine how the mount process should handle it without crashing or repeatedly failing to start; done means the failure is handled gracefully and the user is informed.