When investigating livedata tables with missing entries (e.g. accessing https://<serverurl>/xwiki/rest/liveData/sources/liveTable/entries), the user gets error messages such as "No ExceptionMapper was found, but must be found". We should add more specific details to help narrowing down the issue.
The server side part of the error reporting is done by
XWIKI-16566 , this issue improves the client side reporting by adding a popup error when updated the entries fails, as well as stopping the load bar in case of error.