Improve error for missing segmentation support

[Flamefire]

Fixes #56

[codecov]

Codecov Report

Merging #149 (a2eddc0) into develop (7fe8ec0) will increase coverage by 0.02%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #149      +/-   ##
===========================================
+ Coverage    89.35%   89.38%   +0.02%     
===========================================
  Files          108      108              
  Lines         9435     9446      +11     
===========================================
+ Hits          8431     8443      +12     
+ Misses        1004     1003       -1     

Impacted Files	Coverage Δ
include/boost/locale/boundary/index.hpp	99.18% <100.00%> (+0.42%)	⬆️
test/test_boundary.cpp	97.73% <100.00%> (+0.06%)	⬆️

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