Clean up startup for the pusher (#6558)

* Remove redundant python2 support code

`str.decode()` doesn't exist on python3, so presumably this code was doing
nothing

* Filter out pushers with corrupt data

When we get a row with unparsable json, drop the row, rather than returning a
row with null `data`, which will then cause an explosion later on.

* Improve logging when we can't start a pusher

Log the ID to help us understand the problem

* Make email pusher setup more robust

We know we'll have a `data` member, since that comes from the database. What we
*don't* know is if that is a dict, and if that has a `brand` member, and if
that member is a string.