add basic error handling

hi, pull request includes: 1) code formatting using python black code formatter 2) adds basic error handling if snapshot creation fails: changes the exit code of the script based on how many errors during snapshot creation happen, this makes it easier for callers to report errors during operation based on exit code. Example of successful run: ``` (venv) abi@cefix ~/hcloud-snapshot-as-backup (error)$ python3 snapshot-as-backup.py Snapshot #144915461 (Server #13281368) has been created Snapshot #144915462 (Server #28223848) has been created Snapshot #144915463 (Server #28283699) has been created Snapshot #144915464 (Server #41647785) has been created Snapshot #144915465 (Server #41664956) has been created Snapshot #144915334 (Server #13281368) was successfully deleted Snapshot #144915335 (Server #28223848) was successfully deleted Snapshot #144915336 (Server #28283699) was successfully deleted Snapshot #144915337 (Server #41647785) was successfully deleted Snapshot #144915338 (Server #41664956) was successfully deleted (venv) abi@cefix ~/hcloud-snapshot-as-backup (error)$ echo $? 0 ``` Example of errnous run: ``` (venv) abi@cefix ~/hcloud-snapshot-as-backup (error)$ python3 snapshot-as-backup.py Snapshot for Server #13281368 could not be created: Locked { "error": { "message": "cannot perform operation because server is locked", "code": "locked", "details": null } } [..] (venv) abi@cefix ~/hcloud-snapshot-as-backup (error)$ echo $? 5 ```