Polyfill: Change checks to assertions in adjustEras

[catamorphism]

It would be an internal error to call adjustEras on invalid eras data, so change all these checks to assertions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (03f9057) to head (1cbde9b).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3223      +/-   ##
==========================================
+ Coverage   96.75%   97.44%   +0.68%     
==========================================
  Files          22       22              
  Lines       10398    10391       -7     
  Branches     1859     1824      -35     
==========================================
+ Hits        10061    10125      +64     
+ Misses        289      245      -44     
+ Partials       48       21      -27     

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[ptomato]

One doubt, otherwise LGTM. Thanks!