reword google doc preview error message, fixes CNVS-12778

test plan:
1. attempt to preview the unpreviewable
2. confirm the error message is sensible

Change-Id: Ifba230c83fd46a9f1f603ba4e1b639276d7663d4
Reviewed-on: https://gerrit.instructure.com/34206
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>