Unhelpful error message when scheduling an invalid cron line

[jmettraux]

require 'rufus-scheduler'

s = Rufus::Scheduler.new

s.cron '/15 * * * *' do
  p :nada
end

#s.join

results in

/Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler/jobs.rb:686:in `next_time_from': undefined method `next_time' for nil:NilClass (NoMethodError)
	from /Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler/jobs.rb:696:in `set_next_time'
	from /Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler/jobs.rb:655:in `initialize'
	from /Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler.rb:620:in `new'
	from /Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler.rb:620:in `do_schedule'
	from /Users/jmettraux/w/rufus-scheduler/lib/rufus/scheduler.rb:227:in `cron'
	from t.rb:7:in `<main>'

This is not helpful at all.

As seen in resque/resque-scheduler#649