Fixed the issue of download error modal blocked by download pane when the screen is small

[juzen2003]

• Fixes #1275
• Were any Django, import pipeline, or table_schema files modified? N
  • If YES:
  • Database used (or new import): opus3_test_230109
  • FLAKE8 run on modified code: Y
  • All Django tests pass: Y
  • Code coverage run and still at 100%: Y
• Were any JavaScript or CSS files modified? Y/N
  • If YES:
  • JSHINT run on all affected files: Y/NA
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y/NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): Y/NA
• Was the documentation reviewed for necessary changes or additions? Y/NA
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Description of changes: Update css to make sure the download error modal is not blocked by the download pane when the screen is small.

Known problems: NA