v.io/x/ref/lib/flags: Use verror instead of fmt.Errorf

This CL uses verror instead of fmt.Errorf in v.io/x/ref/lib/flags
Format strings are kept the same as far as  possible (though {1:}{2:} is added to each).

Tested:
	cd v.io
	v23 go test ./...
	v23 test run vanadium-go-test

Change-Id: I0dc4202a4c1b613e55835cc5ccf2891f6557273f
3 files changed