piupartslib/dwke: do not error out on non-utf8 logfiles

Should fix (see e.g. https://piuparts.debian.org/logs/2020/01/11/detect_well_known_errors.txt):

Traceback (most recent call last):
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1911, in <module>
    main()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1851, in main
    section.generate_output(output_directory, section_names, problem_list, web_host)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1660, in generate_output
    self.generate_html()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1587, in generate_html
    failures = dwke_get_failures(self._binary_db, self._problem_list)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1761, in dwke_get_failures
    add_cnt = make_kprs(logdict, kprdict, problem_list)
  File "/srv/piuparts.debian.org/lib/python3/dist-packages/piupartslib/dwke.py", line 252, in make_kprs
    logbody = lb.read()
  File "/usr/lib/python3.7/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf1 in position 26184: invalid continuation byte