Show a sensible error if a corresponding DIRAC user isn't found

  File "/opt/conda/envs/lbapi/lib/python3.9/site-packages/LbAPI/auth.py", line 81, in get_apc
    with RunAsDIRACUser(user.username):
  File "/opt/conda/envs/lbapi/lib/python3.9/site-packages/LbAnalysisProductions/utils.py", line 112, in __init__
    raise LbAPJobFailed(
LbAnalysisProductions.exceptions.LbAPJobFailed: Failed to find a corresponding DIRAC user for lben.
Please make sure you are registered in DIRAC, see the twiki for details:
    https://twiki.cern.ch/twiki/bin/view/LHCb/FAQ/Certificate

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