Added bang functions to client

[scohen]

The Elixir idiom is to have a function that ends with an exclamation point throw an exception, and an unadorned function return {:ok, response} or {:error, reason}. This PR changes Riffed to respoect that idiom. It is a breaking change, and will break all clients upgrading from any version of elixometer < 1.5

Addresses https://github.com/pinterest/riffed/issues/38 @jparise @fire

[fire]

Checks have failed? Weird. Looks.

[fire]

Will you be merging this in?