feat: error page on invalid course key

[ArturGaspar]

Description

Shows a "Not found" page when an invalid course key is in the URL.

Testing instructions

1. Open a course outline page
2. Change the course key in the URL to a non-existent course
3. See the not found page
4. Open an editor page
5. Change the course key (in the first component of the URL after /course/) to a non-existent course
6. See the not found page

Deadline

None

Other information

Private-ref: https://tasks.opencraft.com/browse/BB-8262

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (782fadd) 89.56% compared to head (56d53b4) 89.57%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
+ Coverage   89.56%   89.57%   +0.01%     
==========================================
  Files         495      496       +1     
  Lines        8069     8077       +8     
  Branches     1697     1700       +3     
==========================================
+ Hits         7227     7235       +8     
  Misses        815      815              
  Partials       27       27              

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