MDL-15666 - change all the portfolio plugins and callers to use exceptions

rather than relying on return values (send_package and prepare_package)

move zipping of temporary files into the exporter class so it can be overridden during unit tests

fix a small todo in mahara plugin

mod/assignment/lib.php

@@ -3200,7 +3200,6 @@ class assignment_portfolio_caller extends portfolio_module_caller_base {
         foreach ($this->files as $file) {
             $this->exporter->copy_existing_file($file);
         }
-        return true;
     }
 
     public function get_sha1() {

mod/assignment/type/online/assignment.class.php

@@ -275,7 +275,7 @@ class assignment_online extends assignment_base {
 
     function portfolio_prepare_package($exporter, $userid=0) {
         $submission = $this->get_submission($userid);
-        return $exporter->write_new_file(format_text($submission->data1, $submission->data2), 'assignment.html');
+        $exporter->write_new_file(format_text($submission->data1, $submission->data2), 'assignment.html');
     }
 
     function portfolio_supported_formats() {