return 404 if user does not exist
Revert "return 404 if user does not exist"

This reverts commit 26632ae5baa9426912d9b3882887ec9be6ef1d34.
display appropriate message if wrong username is supplied
pep8
Merge pull request #955 from modilabs/953_user_doesnotexist

display appropriate message if user does not exist when sharing forms - closes #953