Show error when library ids are invalid

[Narfinger]

When the library has some invalid items, the api requests can return an undefined ItemId. In the current version after editing the library it spins endlessly instead of changing or erroring out. This PR gives at least a helpful error message when this happens. This relates to issues such as this: https://github.com/jellyfin/jellyfin/issues/12100#issue-2354924002 https://github.com/jellyfin/jellyfin-web/issues/1012

Two things that might need to change:

• At the moment the error message is hardcoded in english because I do not know how to get the translation involved.
• Perhaps it would be better to catch the undefined ItemId when loading the dialog instead of closing the dialog but I am not sure when this is done.