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/slic3r/GUI/BackgroundSlicingProcess.hpp

@@ -37,6 +37,36 @@ public:
 	PrintBase::SlicingStatus status;
 };
 
+class SlicingProcessCompletedEvent : public wxEvent
+{
+public:
+	enum StatusType {
+		Finished,
+		Cancelled,
+		Error
+	};
+
+	SlicingProcessCompletedEvent(wxEventType eventType, int winid, StatusType status, std::exception_ptr exception) :
+		wxEvent(winid, eventType), m_status(status), m_exception(exception) {}
+	virtual wxEvent* Clone() const { return new SlicingProcessCompletedEvent(*this); }
+
+	StatusType 	status()    const { return m_status; }
+	bool 		finished()  const { return m_status == Finished; }
+	bool 		success()   const { return m_status == Finished; }
+	bool 		cancelled() const { return m_status == Cancelled; }
+	bool		error() 	const { return m_status == Error; }
+	// Unhandled error produced by stdlib or a Win32 structured exception, or unhandled Slic3r's own critical exception.
+	bool 		critical_error() const;
+	// Only valid if error()
+	void 		rethrow_exception() const { assert(this->error()); assert(m_exception); std::rethrow_exception(m_exception); }
+	// Produce a human readable message to be displayed by a notification or a message box.
+	std::string format_error_message() const;
+
+private:
+	StatusType 			m_status;
+	std::exception_ptr 	m_exception;
+};
+
 wxDEFINE_EVENT(EVT_SLICING_UPDATE, SlicingStatusEvent);
 
 // Print step IDs for keeping track of the print state.