Fail gracefully when rendering the project list

Partially addresses #2943 by failing more gracefully from filesystem errors when rendering the project list.

This pull request has two parts: 

1. Relax `NativeFileSystem.readEntries` so that even when some files can't be statted, the list of files that could be statted can still be used by clients. This is accomplished by adding the incomplete list of statted files as an additional, optional argument to the errBack. 

2. Update `ProjectManager._treeDataProvider` to use this additional argument, if possible, when rendering the project list.

See #2943 for more discussion.