[engine] Nicer handling of module exceptions

Do not throw exceptions during exception handling, that causes really
hard-to-interpret error messages.

Instead, log a message and throw outside the exception handler (the code
could do with some cleanup, but is localized enough for now).

see #367

bumblebee/modules/nic.py

@@ -11,14 +11,8 @@ Parameters:
     * nic.format: Format string (defaults to "{intf} {state} {ip} {ssid}")
 """
 
-import logging
-log = logging.getLogger(__name__)
-
-try:
-    import netifaces
-    import subprocess
-except ImportError as e:
-    log.warning("failed to import nic: {}".format(e))
+import netifaces
+import subprocess21
 
 import bumblebee.util
 import bumblebee.input