Use the more detailed errorDetail when handling errorResponses

While I was trying to figure out what went wrong with my Aanmeldproces, I discovered that error messages helping with this, are incomplete.

I was getting
System generating error: acquirer (SE2000) (Directory)

The actual(ly useful) eror message was "MerchantID Unknown", which was suppressed -- because you are not printing 'errorMessage' from the XML, only 'errorDetail'.

Included patch is the first proposal to fix this. You probably have a better overview of all the different error messages, so: whether a semicolon makes sense. Or maybe, if there are situations when it does not make sense to print 'errorMessage'.

The system is Rabobank iDeal Professional, btw. I found earlier (when implementing iDeal on a non Drupal site last year) that its 'errorDetail's are not very descriptive.