feat: ExternalCourseEnrollment page - handle no applicable subsidies

[pwnage101]

Instead of throwing a JS error, return a 404 page when there are absolutely no applicable subsidies for a given course.

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)
• Ensure English strings are marked for translation. See documentation for more details.

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.00%. Comparing base (cdc351c) to head (de9cc06).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1098   +/-   ##
=======================================
  Coverage   86.00%   86.00%           
=======================================
  Files         381      381           
  Lines        7796     7798    +2     
  Branches     1902     1903    +1     
=======================================
+ Hits         6705     6707    +2     
  Misses       1037     1037           
  Partials       54       54           

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[adamstankiewicz]

LGTM! Good catch.