pkg-merge-upstream-to-downstreams: Fix exception handling

Any error during the merge attept currently would fail with:

    During handling of the above exception, another exception occurred:
    Traceback (most recent call last):
      File "/usr/bin/apertis-pkg-merge-upstream-to-downstreams", line 253, in <module>
        main()
      File "/usr/bin/apertis-pkg-merge-upstream-to-downstreams", line 221, in main
        except ErrorReturnCode_2:
    NameError: name 'ErrorReturnCode_2' is not defined

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>