Fix issue273

When an unknown handle graphics element is encountered,
execute the default handling using back-tracking.
Whenever an error occurs, roll back the state of `m2t` to before
encountering the hg-object and continue with the next element.

The original error message is shown for debugging purposes, but
this allows one to run `matlab2tikz` on figures that have
unsupported elements, as was the case in #273.

I'm not that familiar with the `'unknown'` class from Octave, but maybe these cases can be merged. In that case, I would suggest just to leave out the case for `'unknown'`.

This means that instead of getting no output, the output will be partial.