Raise an exception when serializing CQMs with bad labels

[arcondello]

Closes #1358

[codecov]

Codecov Report

Merging #1359 (25ec531) into main (85f5baf) will decrease coverage by 0.01%.
The diff coverage is 87.50%.

❗ Current head 25ec531 differs from pull request most recent head c53c216. Consider uploading reports for the commit c53c216 to get more accurate results

@@            Coverage Diff             @@
##             main    #1359      +/-   ##
==========================================
- Coverage   94.84%   94.84%   -0.01%     
==========================================
  Files          96       96              
  Lines       10171    10175       +4     
==========================================
+ Hits         9647     9650       +3     
- Misses        524      525       +1     

Files Changed	Coverage Δ
dimod/constrained/constrained.py	92.39% <87.50%> (-0.12%)	⬇️

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