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/libslic3r/ModelArrange.hpp

@@ -20,7 +20,7 @@ using VirtualBedFn = std::function<void(arrangement::ArrangePolygon&)>;
 
 [[noreturn]] inline void throw_if_out_of_bed(arrangement::ArrangePolygon&) 
 {
-    throw std::runtime_error("Objects could not fit on the bed");
+    throw Slic3r::RuntimeError("Objects could not fit on the bed");
 }
 
 ArrangePolygons get_arrange_polys(const Model &model, ModelInstancePtrs &instances);