Make CiviMail smarter when handling socket errors

Currently when folks use an external smtp provider like sendgrid / smtp.com etc, new errors are introduced due to various limits enforced by these SaaS providers. This results in emails being bounced, even though the actual email is probably valid. The smtp code in the PEAR packages do not handle the situation and civimail currently processes it as a bounce.

For now if we should treat all socket errors as potential "network errors" rather than smtp errors during a civimail send. This allows us to:

1. Not mark the email as bounced
2. Abort the process if we see a string of these messages in a row

We should log this in CiviCRM.*.log file

Over a period of time, we should improve the detection of errors that are SaaS based over smtp errors

lobo