core: more robust plugins loading:

- avoid stopping loading remaining plugins when a plugin raise an error
    - new error exceptions.MissingModule allows to detect a missing third party module, and to give instructions to get it
    - error during instanciation are catched too

src/core/exceptions.py

@@ -54,6 +54,12 @@ class UnknownGroupError(Exception):
     pass
 
 
+class MissingModule(Exception):
+    # Used to indicate when a plugin dependence is not found
+    # it's nice to indicate when to find the dependence in argument string
+    pass
+
+
 class NotFound(Exception):
     pass