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.cpp

@@ -68,9 +68,10 @@ bool SlicingProcessCompletedEvent::invalidate_plater() const
 	return false;
 }
 
-std::string SlicingProcessCompletedEvent::format_error_message() const
+std::pair<std::string, bool> SlicingProcessCompletedEvent::format_error_message() const
 {
 	std::string error;
+	bool        monospace = false;
 	try {
 		this->rethrow_exception();
     } catch (const std::bad_alloc& ex) {
@@ -78,12 +79,15 @@ std::string SlicingProcessCompletedEvent::format_error_message() const
                               "If you are sure you have enough RAM on your system, this may also be a bug and we would "
                               "be glad if you reported it."))) % SLIC3R_APP_NAME).str());
         error = std::string(errmsg.ToUTF8()) + "\n\n" + std::string(ex.what());
+    } catch (PlaceholderParserError &ex) {
+		error = ex.what();
+		monospace = true;
     } catch (std::exception &ex) {
 		error = ex.what();
 	} catch (...) {
 		error = "Unknown C++ exception.";
 	}
-	return error;
+	return std::make_pair(std::move(error), monospace);
 }
 
 BackgroundSlicingProcess::BackgroundSlicingProcess()