Replaced some of Slic3r::RuntimeError exceptions with Slic3r::SlicingError.

Only Slic3r::SlicingError are now displayed by a notification, other
exceptions are shown by a pop-up dialog.

src/libslic3r/PrintObject.cpp

@@ -139,7 +139,7 @@ void PrintObject::slice()
             }
         });
     if (m_layers.empty())
-        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");    
+        throw Slic3r::SlicingError("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);
 }
 
@@ -427,7 +427,7 @@ void PrintObject::generate_support_material()
             // therefore they cannot be printed without supports.
             for (const Layer *layer : m_layers)
                 if (layer->empty())
-                    throw Slic3r::RuntimeError("Levitating objects cannot be printed without supports.");
+                    throw Slic3r::SlicingError("Levitating objects cannot be printed without supports.");
 #endif
         }
         this->set_done(posSupportMaterial);