Make setCWD() with bad paths safer

Summary: Ref T10853. This isn't a complete fix, but should prevent the most serious mistakes with setCWD() on bad paths.

Test Plan:

• Wrote a script using setCWD().
• Gave it a bad path: got an explicit error.
• Gave it a good path: things worked great.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10853

Differential Revision: https://secure.phabricator.com/D15785