Fix: Environment variables interpolation and exit code

[tarampampam]

Fixed

• Fail the action if an error occurs, instead of just logging the error message

Added

• Environment variables interpolation in run and post commands
• Possibility to determine the shell to use for run and post commands
• Possibility to write the multiline commands using \ separator

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

[jetexe]

LGFM