Add placeholder image thumbnail if image loading error

[FloRinke]

NetBox Edition

NetBox Community

NetBox Version

v4.4.0

Python Version

3.12

Steps to Reproduce

1. Have Cache-Redis without disk persistence (in my case: default directory write-protected by Kubernetes)
2. Attach Image to a suitable object (in my case a location)
3. Go to tab "Images"

Expected Behavior

See an empty box or an error placeholder ("Thumbnail cannot be generated") and a clickable link to the image resource

Observed Behavior

The image file name is shown as text, no preview, no clickable link

In the DOM it's apparent all Tags related to the Thumbnail are missing:

[FloRinke]

In the server log the real issue becomes clear:
redis.exceptions.ResponseError: MISCONF Redis is configured to save RDB snapshots, but it's currently unable to persist to disk. Commands that may modify the data set are disabled, because this instance is configured to report errors during writes if RDB snapshotting fails (stop-writes-on-bgsave-error option). Please check the Redis logs for details about the RDB error.
(But that's not visible to the normal user).
And if you don't know this feature, you also don't know how it's supposed to look like and only see an un-clickable filename.

So I suggest to implement a more graceful handling here. I will try and provide a PR.

[arthanson]

Redis is a required component and needs to be configured correctly, that said it would make sense to have a placeholder image if the image cannot be loaded due to some system error.