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/PrintRegion.cpp

@@ -1,3 +1,4 @@
+#include "Exception.hpp"
 #include "Print.hpp"
 
 namespace Slic3r {
@@ -13,7 +14,7 @@ unsigned int PrintRegion::extruder(FlowRole role) const
     else if (role == frSolidInfill || role == frTopSolidInfill)
         extruder = m_config.solid_infill_extruder;
     else
-        throw std::invalid_argument("Unknown role");
+        throw Slic3r::InvalidArgument("Unknown role");
     return extruder;
 }
 
@@ -40,7 +41,7 @@ Flow PrintRegion::flow(FlowRole role, double layer_height, bool bridge, bool fir
         } else if (role == frTopSolidInfill) {
             config_width = m_config.top_infill_extrusion_width;
         } else {
-            throw std::invalid_argument("Unknown role");
+            throw Slic3r::InvalidArgument("Unknown role");
         }
     }