Show the cherry_picker command when backport failed (GH-30)

Closes python/miss-islington#29

Author: Mariatta

backport/tasks.py

@@ -36,17 +36,29 @@ def backport_task(commit_hash, branch, *, issue_number, created_by, merged_by):
             print(f"pwd: {os.pwd()}, listdir: {os.listdir('.')}")
             util.comment_on_pr(issue_number,
                                f"""{util.get_participants(created_by, merged_by)}, Something is wrong... I can't backport for now.
-                               Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.""")
+                               Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.
+                               ```
+                               cherry_picker {commit_hash} {branch}
+                               ```
+                               """)
     cp = cherry_picker.CherryPicker('origin', commit_hash, [branch])
     try:
         cp.backport()
     except cherry_picker.BranchCheckoutException:
         util.comment_on_pr(issue_number,
                             f"""Sorry {util.get_participants(created_by, merged_by)}, I had trouble checking out the `{branch}` backport branch.
-                            Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.""")
+                            Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.
+                            ```
+                            cherry_picker {commit_hash} {branch}
+                            ```
+                            """)
         cp.abort_cherry_pick()
     except cherry_picker.CherryPickException:
         util.comment_on_pr(issue_number,
                             f"""Sorry, {util.get_participants(created_by, merged_by)}, I could not cleanly backport this to `{branch}` due to a conflict. 
-                            Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.""")
+                            Please backport using [cherry_picker](https://pypi.org/project/cherry-picker/) on command line.
+                            ```
+                            cherry_picker {commit_hash} {branch}
+                            ```
+                            """)
         cp.abort_cherry_pick()