CRM-13493 api don't fatal on duplicate invoice id

----------------------------------------

----------------------------------------
* CRM-13493: Nasty fatal if you try to create a contribution via the api with an invoice id matching an existing
  http://issues.civicrm.org/jira/browse/CRM-13493
Merge pull request #2086 from eileenmcnaughton/CRM_13493

CRM-13493 api don't fatal on duplicate invoice id + don't concantenate errors