Improved error logging in templates.

In sentry looks like:
```
Thumbnail tag failed in template /.../apps/companies/templates/companies/company_layout.html, error at: {% thumbnail company.logo "153x45" as im %}

Stacktrace (most recent call last):

  File "sorl/thumbnail/templatetags/thumbnail.py", line 45, in render
    return self._render(context)
  File "sorl/thumbnail/templatetags/thumbnail.py", line 116, in _render
    file_, geometry, **options
  File "sorl/thumbnail/base.py", line 58, in get_thumbnail
    size = default.engine.get_image_size(source_image)
  File "sorl/thumbnail/engines/convert_engine.py", line 65, in get_image_size
    image['size'] = int(m.group('x')), int(m.group('y'))
```