Status is not set to failed for failed transaction

in CRM-19792, an error was being returned to CiviCRM from Authorize.net because email was not being sent. The error did not result in the Contribution getting a status of failed. Instead, it was left in status of Pending because of https://github.com/civicrm/civicrm-core/blob/master/CRM/Contribute/Form/Contribution/Confirm.php#L2424 silently ignoring non-success return values (and probably elsewhere in code too).

I'm reluctant to change this to deal with errors without a better understanding of the rationale for it as I am not familiar with return codes from payment processors and guess that this ignoring of non-success values is deliberate. For example, perhaps some payment processors callback to say 'not yet done', but later callback to say 'success'. My sense is that error codes from payment processors will be diverse; while all should be logged, there may be a need for each payment processor plugin to map non-success return codes into errors or ignorable other notification codes.

I don't think we should just silently ignore all non-success return codes thinking that site admins should be left to figure out what pending transactions mean.

Eileen McNaughton do you have any insight here into what we should do?