Do not prompt for sudo password in mwscript

If the invoking user does not have passwordless sudo privileges as the
www-data user, fail fast rather than prompting for a sudo password. The
vagrant user has universal passwordless sudo so this should not effect
normal users, but it makes debugging some scripted scenarios easier.

Change-Id: I635f3f51880d7ad36bba7a3138a415f73aa956bd