chezmoi: `output` template function does not cause template rendering to fail

To help find out the issues, the output template function should cause the template rendering to fail if the command which it runs returns a non-zero status code. If someone wants to ignore the status code, it can be worked around with something like output "sh" "-c" "my-command || true".

I took a long time to realize that the chezmoi installation were failing on newly installed systems because curl was not installed before the template runs, then my variables were empty. Instead, I should have been warned by chezmoi that output failed the same way that for example, semverCompare fails if a non-valid string is supplied as input.