Avoid a too broad try...except NameError clause

[matthiask]

Refs #1437

[codecov]

Codecov Report

Merging #1443 (10e1d00) into master (805acf4) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1443      +/-   ##
==========================================
+ Coverage   87.65%   87.95%   +0.29%     
==========================================
  Files          29       29              
  Lines        1588     1586       -2     
  Branches      221      222       +1     
==========================================
+ Hits         1392     1395       +3     
+ Misses        145      141       -4     
+ Partials       51       50       -1     

Impacted Files	Coverage Δ
debug_toolbar/panels/cache.py	84.39% <100.00%> (-0.22%)	⬇️
debug_toolbar/panels/profiling.py	91.07% <0.00%> (+4.46%)	⬆️

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