Change SuccessHandler's function signature

[luckycatx]

Should change middleware jwt's SuccessHandler:

type JWTSuccessHandler func(c echo.Context)
-> type JWTSuccessHandler func(c echo.Context) error

It should be a minor change that will not affect most uses and API. Reason: After obtaining the token, additional condition checks can be performed and the handler chain can be prevented from continuing to execute, instead of requiring an additional middleware.