Fix ValueError handling in custom deserialization

[wyfo]

No description provided.

[codecov-commenter]

Codecov Report

Merging #308 (a542517) into master (36b1b95) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #308      +/-   ##
==========================================
- Coverage   86.00%   86.00%   -0.01%     
==========================================
  Files          67       67              
  Lines        5888     5893       +5     
  Branches     1225     1225              
==========================================
+ Hits         5064     5068       +4     
- Misses        602      604       +2     
+ Partials      222      221       -1     

Impacted Files	Coverage Δ
apischema/deserialization/methods.py	84.76% <100.00%> (-0.04%)	⬇️

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