[libcalamares] Avoid object-slice

g++ warns that error_already_set is polymorphic, and we're catching
by-value (although we don't use that value). Avoid that.

src/libcalamares/PythonJob.cpp

@@ -284,7 +284,7 @@ PythonJob::exec()
             return JobResult::error( message, description );
         }
     }
-    catch ( bp::error_already_set )
+    catch ( bp::error_already_set& )
     {
         QString msg;
         if ( PyErr_Occurred() )