GMN exception when using `updateSystemMetadata()` to change `obsoletes` to an object that is already in an obsolescence chain

[rogerdahl]

View exception: 
Calling view: func_name="get_meta", method="GET", args="('10.5072_rwstg1k34n_2019723204918',)", kwargs="{'allowed_method_list': ['GET']}", url="/v2/meta/10.5072_rwstg1k34n_2019723204918"
Session subjects: CN=urn:node:mnTestRW,DC=dataone,DC=org, authenticatedUser, public
Calling view: func_name="get_meta", method="GET", args="('10.0309_rwstg1k34u_2019810194555',)", kwargs="{'allowed_method_list': ['GET']}", url="/v2/meta/10.0309_rwstg1k34u_2019810194555"
Session subjects: CN=urn:node:mnTestRW,DC=dataone,DC=org, authenticatedUser, public
Calling view: func_name="put_meta", method="PUT", args="()", kwargs="{'allowed_method_list': ['PUT']}", url="/v2/meta"
Session subjects: CN=urn:node:mnTestRW,DC=dataone,DC=org, authenticatedUser, public
Internal exception:
  Name: IntegrityError
  Value: duplicate key value violates unique constraint "app_scienceobject_obsoleted_by_id_key"
  DETAIL:  Key (obsoleted_by_id)=(269) already exists.
  Args: <no args>
  TraceInfo:
    base.py(113)
    contextlib.py(74)
    external.py(388)
    sysmeta.py(133)
    sysmeta.py(528)
    revision.py(125)
    revision.py(411)
    base.py(741)
    base.py(779)
    base.py(851)
    base.py(900)
    query.py(760)
    compiler.py(1429)
    compiler.py(1100)
    utils.py(67)
    utils.py(76)
    utils.py(84)
    utils.py(89)
    utils.py(84)
    Type: <class 'django.db.utils.IntegrityError'>
    Value: duplicate key value violates unique constraint "app_scienceobject_obsoleted_by_id_key"