errors.Is() does not work if multierror.Prefix() is used

[qqshka]

It does not work since go.mod specifies errwrap@v1.0.0 and it does not have Unwrap(), but Unwrap() was added in errwrap@v1.1.0.

I could obviously fix it explicilty by updating versions but fixing it in go.mod wouldn't hurt either.

[dolmen]

#60 would fix that issue.