Improve exception handling for background processes

There were a bunch of places where we fire off a process to happen in the
background, but don't have any exception handling on it - instead relying on
the unhandled error being logged when the relevent deferred gets
garbage-collected.

This is unsatisfactory for a number of reasons:
 - logging on garbage collection is best-effort and may happen some time after
   the error, if at all
 - it can be hard to figure out where the error actually happened.
 - it is logged as a scary CRITICAL error which (a) I always forget to grep for
   and (b) it's not really CRITICAL if a background process we don't care about
   fails.

So this is an attempt to add exception handling to everything we fire off into
the background.

synapse/appservice/scheduler.py

@@ -176,17 +176,20 @@ class _TransactionController(object):
 
     @defer.inlineCallbacks
     def _start_recoverer(self, service):
-        yield self.store.set_appservice_state(
-            service,
-            ApplicationServiceState.DOWN
-        )
-        logger.info(
-            "Application service falling behind. Starting recoverer. AS ID %s",
-            service.id
-        )
-        recoverer = self.recoverer_fn(service, self.on_recovered)
-        self.add_recoverers([recoverer])
-        recoverer.recover()
+        try:
+            yield self.store.set_appservice_state(
+                service,
+                ApplicationServiceState.DOWN
+            )
+            logger.info(
+                "Application service falling behind. Starting recoverer. AS ID %s",
+                service.id
+            )
+            recoverer = self.recoverer_fn(service, self.on_recovered)
+            self.add_recoverers([recoverer])
+            recoverer.recover()
+        except Exception:
+            logger.exception("Error starting AS recoverer")
 
     @defer.inlineCallbacks
     def _is_service_up(self, service):