WIP Refactoring of exceptions:

1) All slicer's exceptions are now derived from Slic3r::Exception.
2) New exceptions are defined for slicing errors.
3) Exceptions are propagated to the Plater to show.
It remains to modify the slicing back-end to throw the new SlicingError
exceptions instead of std::runtime_error and to show the other exceptions
by a message dialog instead of a notification.

src/slic3r/Config/Version.cpp

@@ -324,7 +324,7 @@ std::vector<Index> Index::load_db()
         }
 
     if (! errors_cummulative.empty())
-        throw std::runtime_error(errors_cummulative);
+        throw Slic3r::RuntimeError(errors_cummulative);
     return index_db;
 }