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/GCode/PrintExtents.cpp

@@ -94,7 +94,7 @@ static BoundingBoxf extrusionentity_extents(const ExtrusionEntity *extrusion_ent
     auto *extrusion_entity_collection = dynamic_cast<const ExtrusionEntityCollection*>(extrusion_entity);
     if (extrusion_entity_collection != nullptr)
         return extrusionentity_extents(*extrusion_entity_collection);
-    throw std::runtime_error("Unexpected extrusion_entity type in extrusionentity_extents()");
+    throw Slic3r::RuntimeError("Unexpected extrusion_entity type in extrusionentity_extents()");
     return BoundingBoxf();
 }