feat: return error object from executed command

[josegonzalez]

Description

This is tracked as part of the ExecResult as it might otherwise be shadowed by ctx.Err().

I was seeing the following error that was otherwise swallowed by the execution due to where it was returning:

fork/exec /tmp/sample3357583376: text file busy

Note: the file in question wasn't being properly written/closed before being executed, hence the error message. It took me a while to figure that out.

Closes #20

How Has This Been Tested?

I wrote some tests.

How are existing users impacted? What migration steps/scripts do we need?

This adds a new field so should be mostly backwards compatible

Checklist:

I have:

• updated the documentation and/or roadmap (if required)
• read the CONTRIBUTION guide
• signed-off my commits with git commit -s
• added unit tests

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Aside, should the contributing docs point at the inlets one?