Fix tagging + code quality

Hi there!

Tagging a snapshot would previously fail with this error:

```
ERROR:root:EC2ResponseError: 400 Bad Request
<?xml version="1.0" encoding="UTF-8"?>
<Response><Errors><Error><Code>InvalidParameterValue</Code><Message>Tag keys starting with 'aws:' are reserved for internal use</Message></Error></Errors><RequestID>33b3cb74-f6d7-4e9d-9e97-edf1be8de7e3</RequestID></Response>
```

Perhaps the `aws:(...)` tag exists, because I created my EBS volumes via a CloudFormation script. Anyway, this PR should fix it.

On top of the bug fix mentioned above, I made some code quality improvements (proper spacing, more readable formatting of the logs, variable names, etc.).

Let me know if you're interested in any of these changes and if you're still maintaining this repo (haven't seen any changes since January).