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/Polygon.cpp

@@ -1,5 +1,6 @@
 #include "BoundingBox.hpp"
 #include "ClipperUtils.hpp"
+#include "Exception.hpp"
 #include "Polygon.hpp"
 #include "Polyline.hpp"
 
@@ -16,7 +17,7 @@ Polyline Polygon::split_at_vertex(const Point &point) const
     for (const Point &pt : this->points)
         if (pt == point)
             return this->split_at_index(int(&pt - &this->points.front()));
-    throw std::invalid_argument("Point not found");
+    throw Slic3r::InvalidArgument("Point not found");
     return Polyline();
 }