Correct error messages

Correct error message from "Unable to add userto primary project." to
"Unable to add user to primary project.".

This patch only fix the error messages in the Python file. Because we
have many missing strings in the po files at the moment, we'd better to
update all the strings at once rather than piecemeal.

Fixes bug #1190932

Change-Id: Ib1be37313e70b3c397ca7e15cbfffb9ff523a9b3