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.