Fix: Assert exception messages

[localheinz]

This PR

• asserts exception messages

Follows #1.

[codecov]

Codecov Report

Merging #53 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #53   +/-   ##
=========================================
  Coverage     93.81%   93.81%           
  Complexity       80       80           
=========================================
  Files             8        8           
  Lines           194      194           
=========================================
  Hits            182      182           
  Misses           12       12           

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