bots: fix import logic in STOMP collector's module

The logic regarding importing of the `stomp.py`'s stuff has been fixed:
now the condition of the absence of the `stomp` module (and thus, of the
entire library) would not be confused with the condition of the absence
of only the `stomp.exception` module (which would mean the presence of a
version of that library lacking just the `exception` submodule).

intelmq/bots/collectors/stomp/collector.py

@@ -6,9 +6,10 @@
 
 try:
     import stomp
-    import stomp.exception
 except ImportError:
     stomp = None
+else:
+    import stomp.exception
 
 from intelmq.lib.bot import CollectorBot
 from intelmq.lib.mixins import StompMixin