Gracefully fail when no committers are set in post commit hook

## Feature Request

#### Description
If for some reason the `pre-commit` isn't ran, but the `post-commit` hook is, an error will be thrown. 

#### Example
Example of feature in use in the following format:
```
$ git commit -m "Commit message"

The commit has succeeded, however, guet was not tracking committers. This usually indicates
a missing pre-commit hook.
```