Fix 500 on invalid utf-8 in request

If somebody sends us a request where the the body is invalid utf-8, we should
return a 400 rather than a 500. (json.loads throws a UnicodeError in this
situation)

We might as well catch all Exceptions here: it seems very unlikely that we
would get a request that *isn't caused by invalid json.

synapse/http/servlet.py

@@ -167,7 +167,8 @@ def parse_json_value_from_request(request):
 
     try:
         content = simplejson.loads(content_bytes)
-    except simplejson.JSONDecodeError:
+    except Exception as e:
+        logger.warn("Unable to parse JSON: %s", e)
         raise SynapseError(400, "Content not JSON.", errcode=Codes.NOT_JSON)
 
     return content