Add raise error test in keras_extensions

only one statement is missing to get coverage of 100% in keras_extensions.py

add a test fkt which raises PermissionError in add_callback

added .doing enhancement labels

mentioned in commit f2253185

mentioned in merge request !60 (merged)

mentioned in commit 7742fb56

added Doing label

added merged in Develop label and removed Doing label

mentioned in commit e1bbf647

closed via merge request !59 (merged)