Fixed bad error message with a poor marker.

Found during @WeatherGod's scipy'14 tutorial.
Funny thing about that... the online docs are actually useless here because
they do not have the constants for many of the markers (just the variable
names). My tutorial has a nice table, IMHO:

http://nbviewer.ipython.org/github/WeatherGod/AnatomyOfMatplotlib/blob/master/AnatomyOfMatplotlib-Part2-HowToSpeakMPL.ipynb


On Fri, Jul 11, 2014 at 3:57 PM, Varoquaux <notifications@github.com> wrote:

> In lib/matplotlib/markers.py:
>
> > @@ -241,8 +241,8 @@ def set_marker(self, marker):
> >              try:
> >                  Path(marker)
> >                  self._marker_function = self._set_vertices
> > -            except ValueError:
> > -                raise ValueError('Unrecognized marker style {}'.format(marker))
> > +            except (ValueError, AssertionError):
> > +                raise ValueError('Unrecognized marker style {!r}'.format(marker))
>
> Can we add the list of known marker style? This way, no need to check the
> online documentation. I patched error messages in several places (mostly
> for colormaps) to return both the unkown value and the possible values.
>
> —
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> <https://github.com/matplotlib/matplotlib/pull/3195/files#r14840569>.
>