add a sensible error message for when the root couldn't be resolved

Summary: This could have saved us a bunch of debugging time recently.

Test Plan: `watchman find /` printed a better error message.

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D922516

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extra : amend_source : a33ce31b260b92fea34efc4383237a80b95c743d