Don't allow empty execution in SymPy Live.
completed by: Tristan Hume
mentors: Aaron Meurer, Ondřej Čertík, Hector1618, Stefan Krastanov, Mateusz Paprocki, Matthew Rocklin, Ronan Lamy, Chris Smith, Joachim Durchholz, Vladimir Perić
Please see http://code.google.com/p/sympy/issues/detail?id=2588 for full information on this task.
Please read https://github.com/sympy/sympy/wiki/gci-2011-landing before completing any tasks for SymPy.