Feature request: Better error messages for directories without .gitsecret

[joshrabinowitz]

Currently if you run a git-secret command in an arbitrary directory, you get the error:

% git secret list
'/home/joshr/gitsrc/git-secret/.gitsecret' is ignored. abort.

It would be more user friendly to tell the user a nicer message.

[joshrabinowitz]

See #151

[joshrabinowitz]

This now reports:

if in a git repo without .gitsecret:

git-secret: abort: directory '.gitsecret' does not exist. Use 'git secret init' to initialize git-secret

If in a non-git-repo directory:

git-secret: abort: not in dir with git repo. Use 'git init' or 'git clone', then in repo use 'git secret init'