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.

slic3r.pl

@@ -219,6 +219,8 @@ if (@ARGV) {  # slicing from command line
             $sprint->export_svg;
         } else {
             my $t0 = [gettimeofday];
+            # The following call may die if the output_filename_format template substitution fails,
+            # if the file cannot be written into, or if the post processing scripts cannot be executed.
             $sprint->export_gcode;
             
             # output some statistics