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/ImGuiWrapper.cpp

@@ -927,7 +927,7 @@ void ImGuiWrapper::init_font(bool compress)
     if (font == nullptr) {
         font = io.Fonts->AddFontDefault();
         if (font == nullptr) {
-            throw std::runtime_error("ImGui: Could not load deafult font");
+            throw Slic3r::RuntimeError("ImGui: Could not load deafult font");
         }
     }