Better error message for operations on users that don't exist in linkahead_admin.py

Addressed by !153 (merged):

• Merged

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I tried to receive the roles for a user that does not exist and received the following output by linkahead_admin.py:

$ linkahead_admin.py retrieve_user_roles not.existing.user
Traceback (most recent call last):
  File "(...)/caosdb-pylib/venv/bin/linkahead_admin.py", line 657, in <module>
    sys.exit(main())
             ^^^^^^
  File "(...)/caosdb-pylib/venv/bin/linkahead_admin.py", line 653, in main
    return args.call(args)
           ^^^^^^^^^^^^^^^
  File "(...)/caosdb-pylib/venv/bin/linkahead_admin.py", line 196, in do_retrieve_user_roles
    print(admin._get_roles(username=args.user_name))
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "(...)/caosdb-pylib/src/linkahead/common/administration.py", line 328, in _get_roles
    body = con._http_request(method="GET", path="UserRoles/" + (
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "(...)/caosdb-pylib/src/linkahead/connection/connection.py", line 696, in _http_request
    return self._retry_http_request(method=method, path=path,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "(...)/caosdb-pylib/src/linkahead/connection/connection.py", line 750, in _retry_http_request
    _handle_response_status(http_response)
  File "(...)/caosdb-pylib/src/linkahead/connection/connection.py", line 519, in _handle_response_status
    raise HTTPResourceNotFoundError("This resource has not been found.")
linkahead.exceptions.HTTPResourceNotFoundError: This resource has not been found.

If you don't know ahead of time what is actually wrong, it's difficult to find out. The user probably has no idea what a "resource" is in this case.

Suggestion for a better error message: "User {username} not found/does not exist."