Xero API Interface; better error message displayed if Certificate doesn't support Private Key

Previously if the Client Certificate defined for the Xero API interface did not include a Private Key the following message was displayed "Failed to execute Xero 'GetOrganisations' request (Production). Error creating OAuth Authorisation. Object reference not set to an instance of an object.".

The message now reads "Failed to execute Xero 'GetOrganisations' request (Production). Error creating OAuth Authorisation. Invalid Client Certificate. Private Key not found.".