Do not fail for all LDAP users when user_name_attribute is missing on some entries

[rezib]

Originally posted by @nikolaussuess in https://github.com/rackslab/Slurm-web/discussions/359#discussioncomment-10892343:

The user_name_attribute=uid exists on our LDAP server, and it seems that it is only missing for the administrator user (and exists for all other users). However, when I use user_name_attribute=uid, then I get the following error: MainThread: [ERROR] slurmweb.apps.ldap LDAP error: Unable to extract user uid from user entries I suggest to ignore such users (with a warning) instead of failing for all users. However, setting user_name_attribute=cn works fine.