Change cmd setup to bubble up errors over exiting

[mxygem]

Description

Based on some conversations around testing due to #383, I've done some refactoring of the cobra command section of the code to return errors to the caller and let cobra bubble the error up to the top level call to Execute.

Motivation & context

This change should allow the functions to be more easily testable instead of requiring potentially complicated test setup due to os.Exit() being called by the SUT.

Type of change

• Refactoring/debt (improvement to code design or tooling without changing behaviour)

Checklist:

• I have read the CONTRIBUTING document.
• My change needed additional tests
  • I have added tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[codecov]

Codecov Report

Merging #454 (19b29a8) into main (30de46d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #454   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files          27       27           
  Lines        2186     2186           
=======================================
  Hits         1781     1781           
  Misses        312      312           
  Partials       93       93           

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[vearutop]

LGTM (apart from couple of minor nits)