+ fixed a bug : Case sensitivity of printer's name wasn't check during the adding of a new printer,
as a result in printers list was appeared both of printers (ex. "YuSanka" and "yusanka"), but related file was just one.
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.
