404 and 403 means almost the same for slash identity

[joesong168]

For some implementation, the web server's response to slash identity is hard to change. So is it possible to also tolerant 403?

Forward slash gives 404
Elapsed time (ms): 3684
url: http://iiif.etu.huiyouwenhua.com/image/a/b/full/full/0/default.jpg
got: Expected 404 for status; Got: 403
expected: 404
type: url-check
message: Failed to get random identifier from url http://iiif.etu.huiyouwenhua.com/image/a/b/full/full/0/default.jpg.
warning: false

[glenrobson]

According to the spec the response codes are:

Status	Description
403 Forbidden	The user, authenticated or not, is not permitted to perform the requested operation.
404 Not Found	The image resource specified by identifier does not exist, the value of one or more of the parameters is not supported for this image, or the requested size is greater than the limits specified.

So I think 404 is the correct response but I'm open to allowing 403. What do you think @IIIF/editors?