[IMPR] Show a more informative warning if content.decode() fails
[IMPR] Show a more informative warning if content.decode() fails

Show a more informative warning if content.decode() fails with
UnicodeDecodeError. It is not very helpfull to get a
"Unknown or invalid encoding" message if the reason is the
content e.g. found a b'\xe4\xf6\xfc'

Change-Id: I9d534e002bec33865873b736720723f93a8e01de

wikimedia/pywikibot-core,wikimedia/pywikibot-core