Redirect to error404 when user not found.
Redirect to error404 when user not found.

Closes #3459.

dbalders/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin,TryGhost/Ghost-Admin,acburdine/Ghost-Admin,JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,dbalders/Ghost-Admin,JohnONolan/Ghost-Admin