small bug fixes

Summary:
Two not-so-important bug fixes
         put CmdError in common.mli
         fix find_root_from_absolute_path

Test Plan:
The function works now when you try
'find_root_from_absolute_path path_of_the_root_folder_itself'

Reviewers: pad

Differential Revision: https://phabricator.fb.com/D884664