Use SynapseError 504 for Timeout errors

synapse/util/__init__.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from synapse.api.errors import SynapseError
 from synapse.util.logcontext import PreserveLoggingContext
 
 from twisted.internet import defer, reactor, task
@@ -80,7 +81,7 @@ class Clock(object):
 
         def timed_out_fn():
             try:
-                ret_deferred.errback(RuntimeError("Timed out"))
+                ret_deferred.errback(SynapseError(504, "Timed out"))
             except:
                 pass