Further improvement error reporting with buggy custom G-code sections #1516

1) The macro-processor sanitizes the source code line for invalid UTF-8
   characters. Ideally these invalid characters would be replaced with ?,
   they are just dropped as of now. Better than showing an empty string
   when converting to wxString though.
2) G-code export collects full error message for 1st occurence of an error
   for each custom G-code block.
3) The composite error message now displays the errors collected in 2).
4) The error window is now scaled bigger and the Slicer logo is smaller
   if the text is to be rendered with monospaced font, as the monospaced
   text will not be word wrapped.

src/slic3r/GUI/BackgroundSlicingProcess.hpp

@@ -57,7 +57,8 @@ public:
 	// 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;
+	// 2nd parameter defines whether the output should be displayed with a monospace font.
+	std::pair<std::string, bool> format_error_message() const;
 
 private:
 	StatusType 			m_status;