Improved error reporting of the PlaceholderParser.

The PlaceholderParser is currently used by the GCode.cpp
and by Printer.cpp to generate a new name for the exported G-code or SVG file.
The PlaceholderParser::process() will throw a runtime_error with
a comprehensive error message.
The G-code export will include these error messages into the G-code text
with !!!!!! separators, and the GUI will inform the user, that the G-code
export failed.

xs/xsp/PlaceholderParser.xsp

@@ -14,5 +14,11 @@
         %code%{ THIS->apply_config(*config); %};
     void set(std::string key, int value);
     std::string process(std::string str) const
-        %code%{ RETVAL = THIS->process(str, 0); %};
+        %code%{
+            try {
+                RETVAL = THIS->process(str, 0);
+            } catch (std::exception& e) {
+                croak(e.what());
+            }
+        %};
 };