Abort correctly by calling abort_cherry_pick() (GH-13)

Author: Mariatta

backport/tasks.py

@@ -39,9 +39,9 @@ def backport_task(commit_hash, branch, *, issue_number, created_by, merged_by):
         util.comment_on_pr(issue_number,
                             f"Sorry @{created_by} and @{merged_by}, I had trouble checking out the backport branch."
                             "Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.")
-        cp.abort()
+        cp.abort_cherry_pick()
     except cherry_picker.CherryPickException:
         util.comment_on_pr(issue_number,
-                            f"Sorry, @{created_by} and @{merged_by}, I could not cleanly backport this PR due to a conflict."
-                            "Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.")
-        cp.abort()
+                            f"""Sorry, @{created_by} and @{merged_by}, I could not cleanly backport this PR due to a conflict. 
+                            Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.""")
+        cp.abort_cherry_pick()