Allow to set higher priority to the validation error

[Sam115200]

In case there is already an error on the checkout, for example,

Merchants still want their custom error message to be displayed, which is being set using the validation function. Currently, when there is "no shipping available" error, then the validation function error message is not being displayed.

Please allow to set higher priority to the validation function error message, so merchants can choose to show it, even in the case there are other errors on the checkout. It can be set in the output of the validation function or it can be by default, that validation error message is always displayed, no matter if any other error is being displayed on the checkout or not.