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/PrintObject.cpp

@@ -1,3 +1,4 @@
+#include "Exception.hpp"
 #include "Print.hpp"
 #include "BoundingBox.hpp"
 #include "ClipperUtils.hpp"
@@ -138,7 +139,7 @@ void PrintObject::slice()
             }
         });
     if (m_layers.empty())
-        throw std::runtime_error("No layers were detected. You might want to repair your STL file(s) or check their size or thickness and retry.\n");    
+        throw Slic3r::RuntimeError("No layers were detected. You might want to repair your STL file(s) or check their size or thickness and retry.\n");    
     this->set_done(posSlice);
 }
 
@@ -426,7 +427,7 @@ void PrintObject::generate_support_material()
             // therefore they cannot be printed without supports.
             for (const Layer *layer : m_layers)
                 if (layer->empty())
-                    throw std::runtime_error("Levitating objects cannot be printed without supports.");
+                    throw Slic3r::RuntimeError("Levitating objects cannot be printed without supports.");
 #endif
         }
         this->set_done(posSupportMaterial);