Add friendly error message when GenTests is missing or misspelled

recipe_engine/test.py

Index: recipe_engine/test.py
diff --git a/recipe_engine/test.py b/recipe_engine/test.py
index 68f29c06c2e0048461009d66e24495752ab568aa..e81a12ad1eff37b4053a88e458e1b25808b0d900 100644
--- a/recipe_engine/test.py
+++ b/recipe_engine/test.py
@@ -415,6 +415,9 @@ def get_tests(test_filter=None):
         # TODO(phajdan.jr): move expectation tree outside of the recipe tree.
         expect_dir = os.path.join(root, '%s.expected' % name)
 
+        if not recipe.gen_tests:
+          raise Exception('Missing or misspelled GenTests function.')
+
         # Immediately convert to list to force running the generator under
         # coverage context. Otherwise coverage would only report executing
         # the function definition, not GenTests body.