Add a custom Error 500 template

This PR adds a custom 500 error page, instead of using the GeoNode one.

It contains some duplicated (but simplified) code from ```base.html``` because Django uses a custom ```view``` for 500 errors (see https://docs.djangoproject.com/en/1.5/topics/http/views/#the-500-server-error-view). This doesn't allow the use of ```extends```, ```STATIC_URL``` and so on.

![screenshot from 2014-10-22 11 56 50](https://cloud.githubusercontent.com/assets/1818657/4734359/dbba5d9a-59d2-11e4-87e8-49fdfdfbf064.png)

#251 must be merged first.
Please, add a template comment ( {% comment "optional title" %} comment here {% endcomment %} ) in the ```header.html``` file to remind developers to modify this file when the file is modified accordingly.