[2x] Use a non-zero exit code 3 when skipping build tasks

[caendesilva]

When calling a build task directly, for example with a wrapper like php hyde sitemap:build, the command will now exit with status code 3 instead of 0, in order to signal that something went wrong. If a task is skipped during the main php hyde build command, nothing will change.

The reason behind this is that if you call a specific command like the sitemap build command directly, and it is skipped, then something other than what the user expected happened, so the exit code should reflect that. In comparison: A skipped build task as part of the main site build is not as critical, because in those cases the build task is auxiliary to the expected outcome.

This fixes #1745, please see that issue on how we landed on using exit code 3 when deciding on which non-zero exit code to use.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.97%. Comparing base (b8d30ba) to head (d90dcc7).
Report is 1 commits behind head on 2.x-dev.

Additional details and impacted files

@@            Coverage Diff             @@
##             2.x-dev    #1749   +/-   ##
==========================================
  Coverage      99.97%   99.97%           
  Complexity      1808     1808           
==========================================
  Files            185      185           
  Lines           4788     4788           
==========================================
  Hits            4787     4787           
  Misses             1        1           

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