Warn if extension could not be loaded

[philippjfr]

Fixes #1316

[codecov]

Codecov Report

Merging #1437 into master will decrease coverage by 0.03%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master    #1437      +/-   ##
==========================================
- Coverage   87.93%   87.90%   -0.04%     
==========================================
  Files         135      135              
  Lines       14872    14879       +7     
==========================================
+ Hits        13078    13079       +1     
- Misses       1794     1800       +6     

Impacted Files	Coverage Δ
panel/config.py	45.45% <14.28%> (-1.09%)	⬇️

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