Define behavior of FileSystemFileHandle.getFile() when the underlying file no longer exists.

[mkruisselbrink]

We should define behavior of FileSystemFileHandle.getFile() when the underlying file no longer exists. Currently the spec is pretty much silent on this, we should probably make it clear/explicit that this should reject with a NotFoundError.

[mkruisselbrink]

This is also related to #121, although that is more about what happens to the File if it gets deleted after the File object was created.

[a-sully]

This issue has been ported to the new spec: whatwg/fs#14