Better error message for missing merchant id

[Taylor Briggs]

** (exit) an exception was raised:
    ** (ArgumentError) argument error
        :erlang.bit_size(nil)
        lib/http.ex:54: Braintree.HTTP.process_url/1
        lib/http.ex:2: Braintree.HTTP."request (overridable 1)"/5
        lib/http.ex:44: Braintree.HTTP.request/5
        lib/client_token.ex:23: Braintree.ClientToken.generate/1