fix for issue #50

[kwilliams312]

Issue #50:

Took this route over checking error type return from upstream library due to generic return error.

Added a simple OS directory stat to validate the existence of gitRepoPath.

Tests performed:

• Tested in directory with a git repo initialized

  ken@gnome:~/projects/nogit$ ../autotag/autotag/autotag
  Error initializing:  stat /mnt/c/Users/ken/projects/nogit/.git: no such file or directory

• Tested in an empty directory

  ken@gnome:~/projects/git$ ../autotag/autotag/autotag
  Error initializing:  no stable (non pre-release) version tags found

[joemiller]

thank you @kwilliams312