Have a more helpful error message for past rounds upon encryption in the cli

[AnomalRoil]

Here:

tlock/cmd/commands/encrypt.go

Line 27 in 78dae76

return fmt.Errorf("round %d is not valid anymore", flags.Round)

It is not clear it's failing because the round has already lapsed.
Maybe have instead:
"round %d is in the past, aborting"

[ardan-bkennedy]

Changed