Introduce "FlashErrorAuto" for "XxxErrorAuto" family and simplify the
error handling logic.
* Use ErrorUnwrapForHTML for web error messages
* Use ErrorUnwrapForUser for api error messages
* Make Flash message can take up to about 3000 bytes
Next step: completely refactor "ctx.ServerError"
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Co-authored-by: silverwind <me@silverwind.io>