Better integration with PayPal's address validation

[m4l]

When trying to checkout with a US address leaving the 'state' field empty (not marked as required), I get:

An error occurred communicating with PayPal

After adding the logger:

    'paypal.express': {
        'handlers': ['console', 'file'],
        'propagate': True,
        'level': 'DEBUG',
    },

I can see paypal returning errors like:

The field Shipping Address State is required

or

A match of the Shipping Address City, State, and Postal Code failed.

Could we display those error messages while redirecting to the address input page?

[codeinthehole]

I think so, although it might be tricky to distinguish messages that are suitable for an end-user (like the above one). I'll have a look.