Managed Dependencies: Gracefully handle the situation when the file storage is read-only

[AnatoliB]

On the first function load request, PowerShell worker looks for the latest acceptable dependencies snapshot under D:\home\data\ManagedDependencies, and tries to update the timestamp on a special file (.used) within the snapshot folder. In certain scenarios, this file storage becomes temporarily read-only. Unfortunately, PowerShell worker treats a failure to update the timestamp as a failure to load the function, which makes the app non-functional until the file storage becomes writable again and the worker is restarted.

There is not much we can do when the storage is read-only before the initial managed dependencies installation. However, we can make PowerShell worker tolerate these errors after the first acceptable snapshot is installed. (The only reason to update the timestamp on the .used file is to prevent this snapshot from being purged, but they will not be purged from a read-only file storage anyway, so ignoring these errors is safe.)