fix some network.get_transaction calls

see #4814 (issuecomment-434392195)

electrum/interface.py

@@ -68,9 +68,9 @@ class NotificationSession(RPCSession):
             else:
                 raise Exception('unexpected request: {}'.format(repr(request)))
 
-    async def send_request(self, *args, timeout=-1, **kwargs):
+    async def send_request(self, *args, timeout=None, **kwargs):
         # note: the timeout starts after the request touches the wire!
-        if timeout == -1:
+        if timeout is None:
             timeout = 30
         # note: the semaphore implementation guarantees no starvation
         async with self.in_flight_requests_semaphore:
@@ -108,7 +108,13 @@ class NotificationSession(RPCSession):
 
 
 class GracefulDisconnect(Exception): pass
-class RequestTimedOut(GracefulDisconnect): pass
+
+
+class RequestTimedOut(GracefulDisconnect):
+    def __str__(self):
+        return _("Network request timed out.")
+
+
 class ErrorParsingSSLCert(Exception): pass
 class ErrorGettingSSLCertFromServer(Exception): pass