uow: add "on_exception" lifecycle hook

- When an exception occurs and the unit of work is being rolled back, we
  want to add the possibility to perform some clean-up actions that can
  also be commited to the database.
- The new `on_exception` method is added because we want to keep
  backwards compatibility and also introduce the same "triplet" of
  methods as we have for the "happy path" (`on_register`, `on_commit`,
  `on_post_commit`).

invenio_db/uow.py

@@ -108,6 +108,10 @@ def on_post_commit(self, uow):
         """Called right after the commit phase."""
         pass
 
+    def on_exception(self, uow, exception):
+        """Called in case of an exception."""
+        pass
+
     def on_rollback(self, uow):
         """Called in the rollback phase (after the transaction rollback)."""
         pass
@@ -165,10 +169,10 @@ def __enter__(self):
         """Entering the context."""
         return self
 
-    def __exit__(self, exc_type, *args):
+    def __exit__(self, exc_type, exc_value, traceback):
         """Rollback on exception."""
         if exc_type is not None:
-            self.rollback()
+            self.rollback(exception=exc_value)
             self._mark_dirty()
 
     @property
@@ -193,9 +197,17 @@ def commit(self):
             op.on_post_commit(self)
         self._mark_dirty()
 
-    def rollback(self):
+    def rollback(self, exception=None):
         """Rollback the database session."""
         self.session.rollback()
+
+        # Run exception operations
+        for op in self._operations:
+            op.on_exception(self, exception)
+
+        # Commit exception operations
+        self.session.commit()
+
         # Run rollback operations
         for op in self._operations:
             op.on_rollback(self)