Catch additional error on not existing custom column linked to read column (#2341)

Prevent metadata changes are lost on edit books with errors (#2326)
Better log output
Renamed log message on database delete

cps/render_template.py

@@ -110,8 +110,8 @@ def get_readbooks_ids():
             readBooks = calibre_db.session.query(db.cc_classes[config.config_read_column])\
                 .filter(db.cc_classes[config.config_read_column].value == True).all()
             return frozenset([x.book for x in readBooks])
-        except (KeyError, AttributeError):
-            log.error("Custom Column No.%d is not existing in calibre database", config.config_read_column)
+        except (KeyError, AttributeError, IndexError):
+            log.error("Custom Column No.{} is not existing in calibre database".format(config.config_read_column))
             return []
 
 # Returns the template for rendering and includes the instance name