Symbol import: preemptively error on non-utf8 symbols

Summary:
Put a test somewhere we can --ignore it.

Also fix path tests.

Test Plan: insert good/bad values at various positions.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8353