Solr build_solr_schema: fixed a bug in build_solr_schema. Thanks to mjum...

...bewu for the report!

If you tried to run build_solr_schema with a backend that supports
schema building, but was not Solr (like Whoosh), then you would get an
invalid schema.  This fix raises the ImproperlyConfigured exception
with a proper message.

refers [Issue 687](https://github.com/toastdriven/django-haystack/issues/687)