Implemented naming of the SLA export file based on the output file name

template.

Reworked naming of the plater exports to not use the output file name
template, but to derive the file name from the first printable object's name.

Fixed error handling: Reimpemented the Perl's "eval" blocks
as try / catch blocks.

src/slic3r.cpp

@@ -213,7 +213,7 @@ int main(int argc, char **argv)
                 model.center_instances_around_point(cli_config.print_center);
             }
             if (outfile.empty())
-                outfile = model.objects.front()->input_file + ".gcode";
+                outfile = model.propose_export_file_name() + ".gcode";
             for (auto* mo : model.objects)
                 print.auto_assign_extruders(mo);
             print_config.normalize();