Add better error handling to `buildkite-agent` calls

[technicalpickles]

Over in https://github.com/Gusto/buildkite-builder/issues/64 @muhlba91 reported the upload step failing, but not including any more details.

Under the hood, it's using system which prints the output to stdout, and returns true/false. It'd be nice to have the stdout/stderr when the command fails to figure out what is failing and why.