fix error handler signature

[hnarimiya]

Hello, I have modified the ErrorHandler method signature.

Changed to receive err instead of receiving error message as string.

This is because an error message cannot set an appropriate error object to a response such as api when handling multi errors.

This is an existing breaking change.

However, it seems that there are many cases where the update work does not take that much and there are more merits.

Could you please consider including it in the next minor version?