No error is shown to the user when saving the diagram as SVG or PNG fails

[slauriere]

Steps to reproduce

• Reduce maximum upload size in the current wiki:
  • On page XWiki.XWikiPreferences, edit object XWiki.XWikiPreferences
  • Set Maximum Upload Size field value to 1
• Create diagram
• Add a shape to it
• Hit the Save button

Expected result

• The generated SVG and PNG cannot be stored as attachments since their size exceeds the maximum upload size, so a warning or an error should be shown to the user about the fact the generated images could not be stored.

This expected result assumes that all diagram pages should have either a SVG or a PNG attachment - as an application feature - which seems to be the case indeed since DiagramViewSheet expects such an attachment on export.

Actual result

• Only a success notification is shown with the message "Saved", with no feedback about the generated images storage error.

[slauriere]

Handling a possible fail of the promise returned by saveFilesAsImageAttachments in DiagramEditSheet would fire the display of the expected error:

saveFilesAsImageAttachments().fail(function(e) {
    new XWiki.widgets.Notification(
      $jsontool.serialize($services.localization.render('diagram.editor.saveAsImageAttachmentError')), 'error');
 }).always(function() {
    saveButton.prop('disabled', false).click();
 });

Ready to push this small change to master upon approval unless a pull request is more appropriate.

[mflorea]

@slauriere please create a PR. Thanks.