Prefix Money to Subscription exception modules

[Joe-noh]

I got following error while playing around Money.Subscription functions.

iex(17)> Money.Subscription.change_plan!(s, new, current_interval_started: ~D[2018-01-01]) |>  Money.Subscription.change_plan!(current, current_interval_started: ~D[2018-09-01])
** (UndefinedFunctionError) function Money.Subscription.PlanPending.exception/1 is undefined (module Money.Subscription.PlanPending is not available)
    Money.Subscription.PlanPending.exception("Can't change plan when a new plan is already pending")
    (ex_money) lib/money/subscription.ex:660: Money.Subscription.change_plan!/3

This is not desired behavior and these exceptions have to have Money prefix. Please correct me if I misunderstood. thanks.

(Tests have been passed due to this alias so far?)

[kipcole9]

@Joe-noh Thanks much for the bug report and PR. Merged and released on Hex as version 2.7.1