cubicweb #2538399 [ldapfeed] crash if wrong group specified in the config [done]

only an error in logs would be better

prioritynormal
typebug
done in3.15.9
load0.500
load left0.000
closed by#cf4dacc80976 [ldapfeed] don't crash if one specify an unexisting group in the configuration. Closes #2538399