Unify execution

[devpaul]

Type: cleanup

The following has been addressed in the PR:

• [ ] There is a related issue
• [ ] All code has been formatted with prettier as per the readme code style guidelines
• [ ] Unit or Functional tests are included in the PR

Description:

The code to execute a command's validation and run phases exists in two places: once for the default command and once for other commands. It's too easy to make a change to how a command is executed in one spot and not the other.

This PR unifies command validation and execution for both default and other commands.

[agubler]

@devpaul I think this looks reasonable, just got a conflict that needs resolving.