Improve error messages if Date given instead of DateTime for schedule/{2,3}

[lpil]

The current error message is worse than the default NoClauseError in Elixir because it doesn't tell you which argument is the problem.

We can either improve the message or just let Elixir handle it with a NoClauseError.

[samsondav]

Letting it drop through to a NoClauseError would be fine with me.

[lpil]

Alrighto, I'll do that. Thanks!