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