http://llvm.org/bugs/show_bug.cgi?id=11569

LLDBSwigPythonCallCommand crashes when a command script returns an object 

Add more robustness to LLDBSwigPythonCallCommand.  It should check whether the returned Python object
is a string, and only assign it as the error msg when the check holds.
Also add a regression test.

llvm-svn: 146584