Concert is a model of a concert which contains basic info regarding a concert, taken from dynamoDB.

ErrConcertDoesNotExist is a custom error message to signify the concert with a given ID does not exist.

ErrConcertInPast is a custom error message to signify concert is in past and tickets can no longer be purchased for it.

ErrInvalidConcertData is a custom error message to signify the data from dynamoDB that has been unmarshalled into a struct is incomplete.