fix: do not assume a build script failed completely if there's stderr

[fubhy]

Using experimental features in tailwindcss currently causes build failures in the snowpack-build-script plugin because it considers any stderr output as a critical failure of the script.

I've also opened an issue on the tailwind repository: tailwindlabs/tailwindcss#2522

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[fubhy]

This pull request logs stderr & stdout but changes the error handling behavior so that only exitCode != 0 is considered an actual failure. This gives the underlying scripts some wiggle room for where they log their output (console.warn is an alias for console.error so it logs into stderr). I think tailwind shouldn't use console.warn but also snowpack shouldn't consider stderr to be a critical failure (checking the exit code should be sufficient).

[fubhy]

@FredKSchott I updated the PR accordingly.

[FredKSchott]

LGTM! Errors seem unrelated, will dig into those seperately