Improve an assertion in MacroTest
Improve an assertion in MacroTest

We were only asserting that some code raised an error (ArgumentError),
but we weren't asserting on the message, thus making the test's context
hard to grasp. I added the error message in the assertion and reworded
the test name a bit.

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