PPSR G2B NZ; better error message if a Proxy Server is used

When the PPSR G2B NZ web interface fails and there is a Proxy Server defined sometimes the error message does not include the full error details.

Previously the message would have just stated "Possible causes of this error are: 1. Your Proxy Server settings (ProxyName) are incorrect.".

Now the message will include a message like "Failed to write file attachment Request stream. The error returned was: [ErrorMessage]. Possible causes of this error are: 1. Your Proxy Server settings (ProxyName) are incorrect.".

Note, if the Client Certificate has expired, according to the Tools, User Security, Certificate record, an additional reason that the "Client Certificate has expired." will be shown.