Remove "sweep" label on failure

[sweep-nightly]

This PR addresses the issue of the "sweep" label not being removed when a failure occurs.

The changes made in this PR are in the on_ticket function in the sweepai/handlers/on_ticket.py file. Code has been added to remove the "sweep" label at the points where a failure can occur. This is done by fetching the "sweep" label object using the get_label method and then calling the remove_from_labels method.

Additionally, a case has been handled where the "sweep" label does not exist. This is done by catching the exception raised by the get_label method and logging a message.

This change will ensure that the "sweep" label is removed from the issue when a failure occurs, providing a more accurate representation of the issue's status.

Fixes #152.

To checkout this PR branch, run the following command in your terminal:

git checkout sweep/remove-sweep-label-on-failure_1

[sweep-nightly]

No changes required. The removal of the trailing blank line in sweepai/handlers/on_ticket.py is a good practice for keeping the code clean and consistent.