fix downloads of non-existing ontologies

[alexskr]

fixes #129

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4da3786) 71.85% compared to head (f0e5dc2) 71.85%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #130   +/-   ##
========================================
  Coverage    71.85%   71.85%           
========================================
  Files           52       52           
  Lines         2878     2878           
========================================
  Hits          2068     2068           
  Misses         810      810           

Flag	Coverage Δ
unittests	71.85% <100.00%> (ø)

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