Detect "no file" upload error and, for now at least, ignore it gracefully.

This was triggering on non-AJAX form submissions for new notices, preventing them from being posted when they contained no attachments.

lib/imagefile.php

@@ -79,7 +79,12 @@ class ImageFile
             @unlink($_FILES[$param]['tmp_name']);
             throw new Exception(_('Partial upload.'));
             return;
+         case UPLOAD_ERR_NO_FILE:
+            // No file; probably just a non-AJAX submission.
+            return;
          default:
+            common_log(LOG_ERR, __METHOD__ . ": Unknown upload error " .
+                $_FILES[$param]['error']);
             throw new Exception(_('System error uploading file.'));
             return;
         }