Better debuggability of push failures

Problem/Motivation

The issue is that when an item fails, you have two places to gather the information on the problem. First, you can check salesforce_push_queue table, then the watchdog. By the time you arrive to check the issue, watchdog is perhaps purged. There's backup for sure, but it's getting more and more troublesome.

Proposed resolution

If we'd store the last failure message for the push queue, it would be evident why a single item failed.

API changes

N/A

Data model changes

A new column would be added to salesforce_push_queue table.