refactor: polish http error response

[webup]

Resolves #42

[mergeable]

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[webup]

@benjaminhuo Double confirmed existing logics, current implementation has covered error header part, though we do forget to switch the header key from google's to ours and that's been fixed in this pr.

And since we already have ctx.res and ctx.req in user function to handle response (and they are simple enough), so we are not going to provide further helper function to context object.