strategy/common: add broken attribute to Strategy

This is the first step to broken state tracking for strategies.
A future commit will add a decorator to be used on the the strategy's
transition()/force() method that actually sets the exception and raises
it for subsequent calls

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/strategy/common.py

@@ -26,6 +26,10 @@ class Strategy(Driver):  # reuse driver handling
 
     def __attrs_post_init__(self):
         super().__attrs_post_init__()
+
+        # contains exception that broke the strategy or None
+        self.broken = None
+
         if self.target is None:
             raise BindingError(
                 "Strategies can only be created on a valid target"