Authorize.net does not check for duplicate invoice IDs, so we should before we submit

Unfortunately Authorize.net allows the same invoice ID to be submitted twice. This means that the user can accidentally submit the form twice and thus be billed twice. This patch adds a check for a completed transaction with this invoice ID before submitting to Authorize.net. If one exists an error is thrown.

Also I made the existing "No CURL" error more descriptive (there are a couple of forum posts from confused admins that don't have CURL installed).