Gracefully fail when no committers are set in post commit hook

[chiptopher]

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.