Only amend NoSuchRecipe exceptions instead of all exception with load path.
Only amend NoSuchRecipe exceptions instead of all exception with load path.

This is because the amend function didn't work on some exceptions -- in
particular, it erased the line information from SyntaxError, which was really
terrible UX.

BUG=569103

Review URL: https://codereview.chromium.org/1515333003

luci/recipes-py,luci/recipes-py