typo in sessions file backend can cause NameError
In django/contrib/sessions/backends/file.py, SessionStore.save() refers to "errno.EEXIST" when it means "os.errno.EEXIST". If using the file session backend and set SESSION_FILE_PATH to an unwritable directory (or if there is any other error condition on creating a new session file), this causes a NameError to be raised instead of the correct OSError. It also prevents the correct handling of the (rare) case of a duplicate session ID being generated. Attached patch fixes the typo.