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.
Refactored Arrange interface: remove the union based BedShapeHint, replace it with proper function overloads
WARN: this commit is only intermediate, it does not compile.
