initscr() will exit if there's an error. Try to catch the obvious failure

initscr() will exit if there's an error.  Try to catch the obvious failure
cases if TERM isn't set or is unknown (perhaps we should only check if
unset or empty?)

Skip the test if TERM isn't set.  This seems to occur when running under
buildbot and presumably cron.

For some more info check here:
http://mail.python.org/pipermail/python-checkins/2006-January/048704.html

Will backport if it works.