Mac: Handle failures writing and deleting placeholders when re-expanding folders as part of a projection change

[wilbaker]

Follow up work item for #239, see TODO's in code for specific spots that need to be addressed

[wilbaker]

Re-opening issue. I'm not sure that the changes in #1014 addressed the TODO here:

VFSForGit/GVFS/GVFS.Virtualization/Projection/GitIndexProjection.cs

Line 1439 in 2df2b01

// TODO(Mac): Issue #245, handle failures of WritePlaceholderDirectory and WritePlaceholderFile

I'm thinking if the placeholder write failed we should add that path to ModifiedPaths.dat.

@kewillford what do you think?

[kewillford]

You are right the one that those changes fixed had to do with deleting placeholders that were already on disk not the initial writing of one.