[backport] automatic: Check availability of config file

[m-blaha]

Upstream commit: 13ecc39
RHEL issue: https://issues.redhat.com/browse/RHEL-49743

If a configuration file is explicitly specified on the command line, ensure that it exists and is readable. If the file is not found, notify the user immediately and terminate the process.

This resolves issues where users may run dnf-automatic with unrecognized positional arguments, such as dnf-automatic install.

The most natural approach to handle a non-existing config file would be by catching the exception thrown by the read() method of the libdnf.conf.ConfigParser class. Unfortunately, the Python bindings override the read() method at the SWIG level, causing it to suppress any potentially raised IOError.
For details see this section of the commit
rpm-software-management/libdnf@8f1fedf

def ConfigParser__newRead(self, filenames):
    parsedFNames = []
    try:
        if isinstance(filenames, str) or isinstance(filenames, unicode):
            filenames = [filenames]
    except NameError:
        pass
    for fname in filenames:
        try:
            self.readFileName(fname)
            parsedFNames.append(fname)
        except IOError:
            pass
        except Exception as e:
            raise RuntimeError("Parsing file '%s' failed: %s" % (fname, str(e)))
    return parsedFNames
ConfigParser.read = ConfigParser__newRead

Resolves: https://issues.redhat.com/browse/RHEL-46030

[ppisar]

Good.