Before this commit:
Coupon error on cart would always show "Invalid or Expired coupon code"
instead of a more accurate error (eg: already applied, can't use 2 code..).
Plus it would still show the error on page refresh because of the error being
stored in URL param.
Now:
It will show the exact error and since the error is not stored in URL, it won't
be still displayed on page refresh.
This closes#1601