Only print checkout notices when not reloading the checkout

Any notices that are added during checkout are immediately lost when
the checkout is forced to reload. This commit fixes that by only
printing the notices if the checkout is not being reloaded, otherwise
they’re printed on the page reload and properly displayed to the user.
Merge pull request #5793 from maxrice/fix-disappearing-checkout-notices

Only print checkout notices when not reloading the checkout