Separate message building from reservation creation in cart

[squidgetx]

Inspired by the discussion in #1287, we should attempt to factor out front-end concerns from our backend concerns in the cart module. More specifically, the cart#reserve_all method should not return a fully assembled flash message; this is non-intuitive. The same applies to cart#create_reservation and cart#create_request

It would make more sense for these kinds of methods to return arrays/sets of single error messages or even objects that are later parsed by a view helper or other separate piece of code

[orenyk]

Following #1624 this would now require updating the ReservationCreator service object.