Fix Dropped Generate Error

[alrs]

This fixes a dropped err variable in pkg/goformation/generate. The function signature does not include an error return, so as with other errors in the function this prints a message about the error, prints the error, and exits on status 1.

Because the property variable being operated on is more complicated than a simple string, I marshaled it with the already-imported encoding/json package to make it human-readable. Every field in the struct is exported, so json marshaling works in this case.

[github-actions]

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