[LocalFolder Access] Document whether iterating directory handles and getting directory handle by name could throw

[LuHuangMSFT]

Documentation should be clear about whether these methods could throw when trying to access LocalFolder:

• https://developer.mozilla.org/en-US/docs/Web/API/FileSystemDirectoryHandle/entries
• https://developer.mozilla.org/en-US/docs/Web/API/FileSystemDirectoryHandle/getDirectoryHandle

Throwing an error could help the developer troubleshoot issues like incorrect related_applications manifest data.