Properly extract exception text from Python (fixes issue #24) * exceptions.{h, cc}: Add a new method to properly extract string for a python exception. Also add a new error message if we can't extract the error string. * pycall.cc, pyeval.cc, pyexec.cc: Use the new method to extract string when an exception occurs.

--- a/exceptions.h	Fri Jul 15 00:51:23 2016 -0700
+++ b/exceptions.h	Sat Jul 16 05:50:36 2016 -0700
@@ -159,6 +159,7 @@
   };
 
   bool init_exceptions (void);
+  std::string fetch_exception_message (void);
 }
 
 #endif