Add error message for suggestion elements

In the same way that paper-input has a errorMessage property, our elements:

• nuxeo-user-suggestion
• nuxeo-directory-suggestion
• nuxeo-document-suggestion
• nuxeo-tag-suggestion
• nuxeo-date-picker
• nuxeo-input
• nuxeo-textarea

Should have such property to display a custom message when the value is invalid.