Handle errors on string constraints in custom document form layouts

Steps to reproduce:

• In Studio modeler, configure a custom document.
• To the custom document's schema, add a multi-valued string property with the following constraint:
  • select Web link pattern
  • In the Layout Widget editor, choose a template widget and upload the attached template.
• Add the field to all three layouts and deploy.
• Try creating a custom document in Nuxeo and add https://www.nuxeo.com
  => Error page appears, errors in server.log

Expected result:

• Should be able to create the custom document with a URL in the constrained field.
• If the URL doesn't fit the correct pattern, an error message should appear in the form instead of crashing the UI.