Update vendor (#1414)

vendor/github.com/magefile/mage/mg/errors.go

@@ -0,0 +1,51 @@
+package mg
+
+import (
+	"errors"
+	"fmt"
+)
+
+type fatalErr struct {
+	code int
+	error
+}
+
+func (f fatalErr) ExitStatus() int {
+	return f.code
+}
+
+type exitStatus interface {
+	ExitStatus() int
+}
+
+// Fatal returns an error that will cause mage to print out the
+// given args and exit with the given exit code.
+func Fatal(code int, args ...interface{}) error {
+	return fatalErr{
+		code:  code,
+		error: errors.New(fmt.Sprint(args...)),
+	}
+}
+
+// Fatalf returns an error that will cause mage to print out the
+// given message and exit with the given exit code.
+func Fatalf(code int, format string, args ...interface{}) error {
+	return fatalErr{
+		code:  code,
+		error: fmt.Errorf(format, args...),
+	}
+}
+
+// ExitStatus queries the error for an exit status.  If the error is nil, it
+// returns 0.  If the error does not implement ExitStatus() int, it returns 1.
+// Otherwise it retiurns the value from ExitStatus().
+func ExitStatus(err error) int {
+	if err == nil {
+		return 0
+	}
+	exit, ok := err.(exitStatus)
+	if !ok {
+		return 1
+	}
+	return exit.ExitStatus()
+}