[IMP] runbot_merge: non-empty fallback on fast-forward failure

Up till now if an FF failed with an exception having neither cause nor
context the cancel reason would be an empty string.

Fallback on stringifying the exception itself as a last resort.

runbot_merge/models/pull_requests.py

@@ -1687,7 +1687,7 @@ class Stagings(models.Model):
                 )
                 self.write({
                     'state': 'ff_failed',
-                    'reason': str(e.__cause__ or e.__context__ or '')
+                    'reason': str(e.__cause__ or e.__context__ or e)
                 })
             else:
                 prs = self.mapped('batch_ids.prs')