Catch symbolic relational error in Sum.evalf().

Partly fixes issues 3229. The remaining part has to do
with the .evalf() call when printing. This happens
because Sum(f(x), (x, 1, 3)).is_number is True which,
according to current definitions, seems correct.

http://code.google.com/p/sympy/issues/detail?id=3229
Merge pull request #2 from jrioux/concrete

Catch symbolic relational error in Sum.evalf().