[core/input] Ignore errors during callbacks

If a callback command cannot be executed, do not terminate the whole
bar. Furthermore, if such a failure occurs, try to fall back to a
globally (i.e. non widget-specific) command, so that callbacks
registered in the engine itself (such as mouse-wheel scrolling) still
function properly.

fixes #166

bumblebee/input.py

@@ -126,9 +126,14 @@ class I3BarInput(object):
 
         if cmd is None:
             return
+        try:
             if callable(cmd):
                 cmd(event)
             else:
                 bumblebee.util.execute(cmd, False)
+        except Exception:
+            # fall back to global default
+            if not "__fallback" in event:
+                return self.callback({ "name": None, "instance": None, "__fallback": True, "button": event["button"] })
 
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