refactor x < 1 error checks: optimized inlining and less duplication

[nsajko]

Split from PR #141.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.08%. Comparing base (52e004e) to head (34f7349).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
+ Coverage   90.72%   91.08%   +0.36%     
==========================================
  Files           1        1              
  Lines         399      404       +5     
==========================================
+ Hits          362      368       +6     
+ Misses         37       36       -1     

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[nsajko]

I see this as a straight improvement, and I do not see it as possibly controversial, and my other PR depends on this. So I will be bold and merge right away.