Exception not catched for PostgreSQL when attaching DIDs twice.

[hahahannes]

Motivation

While running the test of this PR, I encountered an exception only with PostgreSQL as database, that is not catched. It raised after tryting to attach a dataset to a container twice.

rucio add-container mock:container
rucio add-dataset mock:dataset
rucio attach mock:container mock:dataset
rucio attach mock:container mock:dataset

ERROR	An unknown exception occurred.
Details: [u'(psycopg2.IntegrityError) duplicate key value violates unique constraint "CONTENTS_PK"\nDETAIL:  Key (scope, name, child_scope, child_name)=(mock, co, mock, da) already exists.\n']

With other databases a DuplicateContent gets thrown.

Modification

I would add it to the pattern matching of the IntegrityError here