Render repr(field), not repr(BoundField)

It's somewhat meaningless to show an error about the BoundField not being able to be rendered because no matching block was found, especially since it was the ``field.field`` that was examined, not the BoundField.

Thus, the error message changes from:

    TemplateSyntaxError: Could not find widget for field: <django.forms.forms.BoundField object at 0x1112dbd10>

to:

    TemplateSyntaxError: Could not find widget for field: <django.forms.fields.CharField object at 0x103976150>