revpanelwidget: correct import of hg errors

This was giving a traceback on shutdown like the following:

NameError: global name 'error' is not defined
Traceback (most recent call last):
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\repowidget.py", line 268, in revision_selected
self.revDetailsWidget.revision_selected(rev)
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\revdetailswidget.py", line 267, in revision_selected
self.revpanel.update(ctx.rev())
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\csinfo.py", line 529, in update
markups = self.get_markup(item)
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\csinfo.py", line 435, in get_markup
return self.info.get_markup(item, self, self.ctx, self.custom, **kargs)
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\csinfo.py", line 381, in get_markup
value = self.get_data(item, *args)
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\csinfo.py", line 324, in get_data
return custom['data'](widget, item, ctx)
File "e:\george\tortoisehg\thg\tortoisehg\hgqt\revpanelwidget.py", line 82, in data_func
except (error.LookupError, error.RepoLookupError, error.RepoError):
NameError: global name 'error' is not defined