main: report the guessed reason if system("sort") is failed

[masatake]

$ ~/bin/ctags -o - -R ~/var/ctags-github/main |readtags __NR_read
ctags: cannot sort tag file: system ("sort -u") exited with 13 (Broken pipe?)

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: -0.01% ⚠️

Comparison is base (c7911c4) 83.05% compared to head (2789d62) 83.04%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3788      +/-   ##
==========================================
- Coverage   83.05%   83.04%   -0.01%     
==========================================
  Files         227      227              
  Lines       55200    55201       +1     
==========================================
- Hits        45844    45841       -3     
- Misses       9356     9360       +4     

Files Changed	Coverage Δ
main/sort.c	83.63% <50.00%> (ø)

... and 2 files with indirect coverage changes

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