[1.4.x] Removed try-except in django.db.close_connection()

The reason was that the except clause needed to remove a connection
from the django.db.connections dict, but other parts of Django do not
expect this to happen. In addition the except clause was silently
swallowing the exception messages.

Refs #19707, special thanks to Carl Meyer for pointing out that this
approach should be taken.
[1.4.x] Fixed #19815 - Removed an unused import in tutorial 3.

Thanks pedro.calcao@ for the report.