Test for AuxException cases

[jakobnissen]

Also make sure that AuxExceptions are thrown more often, where possible

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[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.57%. Comparing base (4ba1617) to head (959c69a).

Files with missing lines	Patch %	Lines
src/bam.jl	50.00%	1 Missing ⚠️
src/sam.jl	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
+ Coverage   81.60%   88.57%   +6.97%     
==========================================
  Files           5        5              
  Lines         625      639      +14     
==========================================
+ Hits          510      566      +56     
+ Misses        115       73      -42     

Flag	Coverage Δ
unittests	88.57% <92.00%> (+6.97%)	⬆️

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