Better warning message for when guet set is called in a non-git folder

[chiptopher]

Feature Request

Description

Calling guet set in a folder that doesn't have a .git folder causes an the global error message to appear. Issue #29 addresses this issue. In the meantime, an error message that describes the situation would be good.

Example

# in the root directory
$ cd api/
$ guet set n1 n2
You are not in a directory with a git folder. Change back
to your project's root directory.