Checkmk Werk 11915: Fix error messages on check plugin page if check or catalog entry is missing

ID: 11915 Title: Fix error messages on check plugin page if check or catalog entry is missing Component: Multisite Level: 1 Class: Bug fix Version: 2.1.0i1 Missing checks of check plugins with manual page lead to an undefined error message. If such an error occurs now, a meaningful error message is displayed. Also, in case of missing catalog entries in the manual page, the catalog is set to "unsorted" correctly now.