func (s Stack) format(sep string, invert bool) []string {
	var str []string
	for _, f := range s {
		if invert {
			str = append(str, f.format(sep))
		} else {
			str = append([]string{f.format(sep)}, str...)
		}
	}
	return str
}
format returns an array of formatted stack frames.
package eris
import (
	"fmt"
	"runtime"
	"strings"
)
type Stack []StackFrame