RFE: provide hint if permissions do no allow any journal files to be opened

Start with the Python systemd.journal.Reader() entry point and reproduce the CentOS permission case using this_boot(), log_level(), and get_next(). The change is complete when reading with no accessible journal files reports the insufficient-permissions condition instead of silently yielding empty results; the payload names no source file or test.

Reading from journald without permission to do so should throw an exception but doesn't?

build:~$ cat /etc/os-release 
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

build:~$ 
build:~$ groups 
developers
build:~$ 
build:~$ journalctl --since "1 hour ago"
Hint: You are currently not seeing messages from other users and the system.
      Users in the 'systemd-journal' group can see all messages. Pass -q to
      turn off this notice.
No journal files were opened due to insufficient permissions.
build:~$ 
build:~$ python3.6
Python 3.6.3 (default, Jan  4 2018, 16:40:53) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-16)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 
>>> 
>>> from systemd import journal
>>> j = journal.Reader()
>>> j.this_boot()
>>> j.log_level(journal.LOG_DEBUG)
>>> j.get_next()
{}
>>> for entry in j:
...     print(entry)
... 
>>>