Guzzle exceptions in Whitespace provider are not caught

Pushing an attempt at a fix, but outputting an error message when we catch an exception contradicts the error message in twig:

<p class="alert alert-info">No properties found for postcode {{ noresults }}. Please check this is a valid postcode and search again.</p>

as both will show in the case of a Guzzle exception.

Not sure how best to fix this.

I should move the catching to a helper method, as this should happen further on too.

Added a helper for making SOAP calls, which handles SOAP errors and re-throws them.

Thanks @joachim, reviewed and tested, looks good to me.