Improve D-Bus timeout error handling (#5664)

* Improve D-Bus timeout error handling

Typically D-Bus timeouts are related to systemd activation timing out
after 25s. The current dbus-fast timeout of 10s is well below that
so we never get the actual D-Bus error. This increases the dbus-fast
timeout to 30s, which will make sure we wait long enought to get the
actual D-Bus error from the broker.

Note that this should not slow down a typical system, since we tried
three times each waiting for 10s. With the new error handling typically
we'll end up waiting 25s and then receive the actual D-Bus error. There
is no point in waiting for multiple D-Bus/systemd caused timeouts.

* Create D-Bus TimedOut exception

supervisor/exceptions.py

@@ -403,7 +403,11 @@ class DBusParseError(DBusError):
 
 
 class DBusTimeoutError(DBusError):
-    """D-Bus call timed out."""
+    """D-Bus call timeout."""
+
+
+class DBusTimedOutError(DBusError):
+    """D-Bus call timed out (typically when systemd D-Bus service activation fail)."""
 
 
 class DBusNoReplyError(DBusError):