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/GUI/wxExtensions.cpp

@@ -436,7 +436,7 @@ wxBitmap create_scaled_bitmap(  const std::string& bmp_name_in,
 
     if (bmp == nullptr) {
         // Neither SVG nor PNG has been found, raise error
-        throw std::runtime_error("Could not load bitmap: " + bmp_name);
+        throw Slic3r::RuntimeError("Could not load bitmap: " + bmp_name);
     }
 
     return *bmp;