Abort on error

On a (debian) system without sudo, lots of messages rush by, and it's not obvious was fails.
With this change, the log looks like:

    $ bash install-beef
    bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
    ======================================
               BeEF Installer
    ======================================

    CAUTION: This installation script will install a number of BeEF dependencies including the Ruby-RVM environemnt and it's dependencies.

    In rare cases, this may lead to unexpected behaviour or package conflicts on some systems.

    Are you sure you wish to continue (Y/n)?

    Detecting OS..
    Debian/Ubuntu Detected
    Installing Prerequisite Packages..
    install-beef: line 74: sudo: command not found

which is far more informative.