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/rest/media/v1/storage_provider.py

@@ -18,7 +18,7 @@ from twisted.internet import defer, threads
 from .media_storage import FileResponder
 
 from synapse.config._base import Config
-from synapse.util.logcontext import preserve_fn
+from synapse.util.logcontext import run_in_background
 
 import logging
 import os
@@ -87,7 +87,12 @@ class StorageProviderWrapper(StorageProvider):
             return self.backend.store_file(path, file_info)
         else:
             # TODO: Handle errors.
-            preserve_fn(self.backend.store_file)(path, file_info)
+            def store():
+                try:
+                    return self.backend.store_file(path, file_info)
+                except Exception:
+                    logger.exception("Error storing file")
+            run_in_background(store)
             return defer.succeed(None)
 
     def fetch(self, path, file_info):