Improve error message for invalid arguments in Protocol.def/1
Improve error message for invalid arguments in Protocol.def/1

gfvcastro/elixir,joshprice/elixir,elixir-lang/elixir,antipax/elixir,michalmuskala/elixir,beedub/elixir,lexmag/elixir,gfvcastro/elixir,kimshrier/elixir,lexmag/elixir,ggcampinho/elixir,pedrosnk/elixir,kelvinst/elixir,beedub/elixir,pedrosnk/elixir,ggcampinho/elixir,kelvinst/elixir,antipax/elixir,kimshrier/elixir