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.

synapse/push/pusherpool.py

@@ -232,7 +232,6 @@ class PusherPool:
             Deferred
         """
         pushers = yield self.store.get_all_pushers()
-        logger.info("Starting %d pushers", len(pushers))
 
         # Stagger starting up the pushers so we don't completely drown the
         # process on start up.
@@ -245,7 +244,7 @@ class PusherPool:
         """Start the given pusher
 
         Args:
-            pusherdict (dict):
+            pusherdict (dict): dict with the values pulled from the db table
 
         Returns:
             Deferred[EmailPusher|HttpPusher]
@@ -254,7 +253,8 @@ class PusherPool:
             p = self.pusher_factory.create_pusher(pusherdict)
         except PusherConfigException as e:
             logger.warning(
-                "Pusher incorrectly configured user=%s, appid=%s, pushkey=%s: %s",
+                "Pusher incorrectly configured id=%i, user=%s, appid=%s, pushkey=%s: %s",
+                pusherdict["id"],
                 pusherdict.get("user_name"),
                 pusherdict.get("app_id"),
                 pusherdict.get("pushkey"),
@@ -262,7 +262,9 @@ class PusherPool:
             )
             return
         except Exception:
-            logger.exception("Couldn't start a pusher: caught Exception")
+            logger.exception(
+                "Couldn't start pusher id %i: caught Exception", pusherdict["id"],
+            )
             return
 
         if not p: