Add special handling for the MS_MAX_CONCURRENT_REQ error

Problem/Motivation

We're using this module on a couple forms and noticed in the logs that the following error code is regularly being returned by the VIES service:

MS_MAX_CONCURRENT_REQ

As you can find in the documentation of the WSDL of this service:

MS_MAX_CONCURRENT_REQ: Your Request for VAT validation has not been processed; the maximum number of concurrent requests for this Member State has been reached. Please re-submit your request later or contact TAXUD-VIESWEB@ec.europa.eu for further information": Your request cannot be processed due to high traffic towards the Member State you are trying to reach. Please try again later.

I propose we inform the user about this through an error message and ask them to try again in a couple minutes, instead of telling them they entered a wrong VAT number. This last part also applies to other error codes returned by the VIES service, in that case we shouldn't tell users they entered a wrong VAT number.

Steps to reproduce

This happens quite randomly, it's hard to reproduce.

Proposed resolution

Show more informative validation messages.