Add proper message if branch does not exist
Remove unnecessary null_value method in GetGpa class
Rename branch_gpa_is_nil method to branch_not_found in CompareGpa class
Move not_found method from CompareGpa to GetGpa class
Add not_found method stub in cli_spec
Move exception checks in special class
Merge pull request #35 from fs/add-proper-message-if-branch-does-not-exist

Add proper message if branch does not exist