Misleading error message if config.yaml is not readable

[tehplague]

If pwmgr is being run as an unprivileged user it complains about a nonexistant /etc/pwmgr/config.yaml file. However, the file might exist but might be inaccessible at the same time, then leading to the same error message.

Instead, pwmgr should tell the user that he is not allowed to access the configuration file.