Improve error handling in templating and conditionals

[zachariahmiller]

Describe what should be investigated or refactored

Currently when Maru errors do to an invalid template, even in the if: blocks it throws a pretty misleading error about bad substitution. This should be updated to be more clear.

Links to any relevant code

(optional) i.e. - https://github.com/defenseunicorns/maru-runner/blob/main/README.md?plain=1#L1

Additional context

Add any other context or screenshots about the technical debt here.