Parse incorrectly shaped validation errors

[bhelx]

Resolves #284
Resolves #281

Some coupon validation errors are shaped like normal field errors, for instance:

<?xml version="1.0" encoding="UTF-8"?>
<error>
  <symbol>subscription_must_change</symbol>
  <description>You must change the subscription in order to redeem a coupon.</description>
</error>

instead of what would be expected:

<?xml version="1.0" encoding="UTF-8"?>
<errors>
  <error field="subscription.coupon_code" symbol="subscription_must_change">You must change the subscription in order to redeem a coupon.</error>
</error>

Example code:

$active_subscription = '3b1c9a5c19f46a8e205abe409ebe3c01';

try {
  $subscription = Recurly_Subscription::get($active_subscription);
  $subscription->coupon_code = 'j32yaysm6g';

  $subscription->quantity = 2;
  $subscription->updateImmediately();

  print var_dump($subscription);
} catch (Recurly_ValidationError $e) {
  // prints all the error messages summed up
  print $e->getMessage() . "\n"; // You must change the subscription in order to redeem a coupon.

  // you can also dig through the individual errors, in this case there is only one
  foreach ($e->errors as $fieldErr) {
    print $fieldErr->symbol . "\n"; // subscription_must_change
    print $fieldErr->description . "\n"; // You must change the subscription in order to redeem a coupon.
  }
}