become: Make more user-friendly

Currently, if you try to become a tool that you are not a member of,
the error message is rather cryptic:

| scfc@tools-login:~$ become coursestats
| sudo: sorry, a password is required to run sudo
| scfc@tools-login:~$

This change makes the error message more meaningful and also addresses
the common issue when a user has been made a member of the group but
his login session predates that change.

Bug: 68156
Change-Id: I801608bf5cf47d339343838987240336cb012998