raise error when recording migrants

[GertjanBisschop]

Solves #2190.

[codecov]

Codecov Report

Merging #2199 (83a2b9c) into main (f9dd5bd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2199   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files          20       20           
  Lines       11254    11255    +1     
  Branches     2297     2297           
=======================================
+ Hits        10293    10294    +1     
  Misses        523      523           
  Partials      438      438           

Flag	Coverage Δ
C	91.46% <100.00%> (+<0.01%)	⬆️
python	98.68% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
msprime/ancestry.py	98.66% <100.00%> (+<0.01%)	⬆️

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[jeromekelleher]

LGTM!