Improve exceptions usage in specs:

* Throw symbols instead of raising exceptions to test code evaluation,
  this will reduce "bad" usages of exceptions, symbols seems a little
  better for now;
* Add a specific exception to be used in specs for testing behaviour
  when an exception is raised from a recipe.

spec/fixtures/recipes/error.rb

@@ -1 +1 @@
-raise 'error from recipe'
+raise SomeErrorInRecipe, 'some recipe error'

spec/fixtures/recipes/throw.rb

@@ -0,0 +1 @@
+throw :recipe_code