Added check for git

Notifies user if git is not installed. It still works, but has slightly reduced
functionality as notes cannot be versioned.
Removed all refs to note_dir_exists

Removed this function in commit 848b14b776bfd4df4b18496d279a5e91765c237e as the
functionality is handled in the main function now.
Minor tweaking of git not-found error text

Just added some new lines. Updated the text a bit and added NOTE ot the
beginning to make it stand out a bit more.