Fix UnboundLocalError in find_recipe.
Fix UnboundLocalError in find_recipe.

It can happen instead of NoSuchRecipe if users pass some nonsense instead of
a recipe name.

R=iannucci@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2983743002

luci/recipes-py,luci/recipes-py