Log error on duplicate webext-messenger execution

[fregante]

What does this PR do?

The actual changes code are in the following PR

• Log error in case of multiple injections webext-messenger#176

Reviewer Tips

This PR doesn't yet include the attached PR, but you can see the result in the demo below

Discussion

As found in pixiebrix/webext-messenger#88, this is the easiest way to make the error visible without losing DX and causing further issues.

Demo

In the first commit of this PR, you can see the code triggering the error:

• on CI
• at runtime, even for files not covered by CI:

Checklist

• Add tests: I clarified the CI checks
• Designate a primary reviewer: @BLoe

Potentially related

• Target for RUN_BRICK not found when targeting the window: top tab #6807

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6823904) 70.41% compared to head (8e6f052) 70.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6856   +/-   ##
=======================================
  Coverage   70.41%   70.42%           
=======================================
  Files        1195     1195           
  Lines       37140    37140           
  Branches     6968     6968           
=======================================
+ Hits        26153    26154    +1     
+ Misses      10987    10986    -1     

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[github-actions]

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