Change "sql: no rows in result set"

[farazfazli]

I propose changing "sql: no rows in result set" for yet to be created pages to something more understandable by the average user, such as "Page not found, add a custom slug to create it!"

[tomkwok]

This issue is already listed in #31: use error.hbs in theme when 500 / 404 / etc. is returned (no more sql: no rows in result set and HTTP 500 response).
And I also propose changing HTTP response code from 500 to 404. "Page not found" isn't an "internal server error" anyway.