Maru doesn't error on go templating failures

[corang]

Steps to reproduce

1. have an action cmd that references a non existent var or input via the ${{ .inputs.x }} syntax

Expected result

maru alerts the user that the reference they've made doesn't exist

Actual Result

maru silently fails go templating and passes the literal ${{ .inputs.x }} to the shell where it errors, usually with "bad substitution"