Raise a PagureEncodingException when none of the encoding guessed worked
In pagure.doc_utils.convert_readme() we try guessing the encoding of the
file, then we iterate over all these encoding and try to decode the file
in it.
If all the encoding fails, we raise an exception, it used to be a generic
PagureException, it will now be a PagureEncodingException inheriting from
both PagureExpception and ValueError