subclasses of ParserBase must override error()

it seems that the `HTML2Text` class is a subclass of `_markupbase.ParserBase`, which requires subclasses to implement their own `error()` method. When I am trying to parse some HTML that is throwing an error (I'll have to instrument my code to recover an example), it leads to the following stack trace: (f

```
File "/opt/webapp/foo/utilities.py" in html_to_text
74. text = converter.handle(text)

File "/usr/local/lib/python3.6/dist-packages/html2text/__init__.py" in handle
129. self.feed(data)

File "/usr/local/lib/python3.6/dist-packages/html2text/__init__.py" in feed
126. super().feed(data)

File "/usr/lib/python3.6/html/parser.py" in feed
111. self.goahead(0)

File "/usr/lib/python3.6/html/parser.py" in goahead
179. k = self.parse_html_declaration(i)

File "/usr/lib/python3.6/html/parser.py" in parse_html_declaration
264. return self.parse_marked_section(i)

File "/usr/lib/python3.6/_markupbase.py" in parse_marked_section
149. sectName, j = self._scan_name( i+3, i )

File "/usr/lib/python3.6/_markupbase.py" in _scan_name
391. % rawdata[declstartpos:declstartpos+20])

File "/usr/lib/python3.6/_markupbase.py" in error
34. "subclasses of ParserBase must override error()")

Exception Type: NotImplementedError
Exception Value: subclasses of ParserBase must override error()

```

- Version by `html2text --version`

```
$ html2text --version
2019.9.26
```

- Test script

I will have to instrument my code to find an example. Coming soon!

- Python version `python --version`

```
$ python --version
Python 3.6.10
```