More informative errors instead of assertions

[fjarri]

No description provided.

[codecov-commenter]

Codecov Report

Merging #3 (0f5a304) into master (3117099) will decrease coverage by 1.65%.
The diff coverage is 31.81%.

@@            Coverage Diff             @@
##           master       #3      +/-   ##
==========================================
- Coverage   77.55%   75.89%   -1.66%     
==========================================
  Files           8        8              
  Lines         490      502      +12     
==========================================
+ Hits          380      381       +1     
- Misses        110      121      +11     

Impacted Files	Coverage Δ
pons/client.py	90.59% <0.00%> (-0.79%)	⬇️
pons/types.py	76.27% <35.00%> (-6.91%)	⬇️

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