Handle non-zero exit status when formatting using shfmt

[chris-reeves]

Non-zero exist statuses from shfmt weren't checked for or handled. As a result the (empty) output from shfmt was returned as the formatted document. An exception is now thrown when a non-zero exit status is encountered.

Fixes #1162

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.21%. Comparing base (1fc9819) to head (921fa36).
Report is 1 commits behind head on main.

❗ Current head 921fa36 differs from pull request most recent head 522a5d6. Consider uploading reports for the commit 522a5d6 to get more accurate results

Files	Patch %	Lines
server/src/shfmt/index.ts	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1163      +/-   ##
==========================================
+ Coverage   81.19%   81.21%   +0.02%     
==========================================
  Files          29       29              
  Lines        1409     1411       +2     
  Branches      334      335       +1     
==========================================
+ Hits         1144     1146       +2     
  Misses        218      218              
  Partials       47       47              

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