verror should handle %w

[cosnicolaou]

verror.Errorf does not currently understand %w and leads to ugly output when it's used, see the example code below outputs:

err: t: cause: %!w(*errors.errorString=&{an error})
cause: an error

package main

import (
        "errors"
        "fmt"

        "v.io/v23/verror"
)

func main() {
        var (
                cause = errors.New("an error")
                err   = verror.ErrInternal.Errorf(nil, "cause: %w", cause)
        )
        // The printing does not handle the "%w" nicely...
        fmt.Printf("err: %v\n", err)

        // ... even though the wrapping is done correctly.
        fmt.Printf("cause: %v\n", errors.Unwrap(err))
}