1. Avoid print large string in error log
If user load a unqualified large string, the all string will be saved in error log,
so the error log is too big that can not be shown be using `show load warnings on "url"`.
Err: `Got packet bigger than 'max_allowed_packet' bytes`
2. Remove duplicate help doc
Do not allow doc with same title, or error thrown when starting FE:
`java.lang.IllegalArgumentException: Multiple entries with same key:`