More explicit error message when value in Advanced mode configuration's EL expression cannot be evaluated

When the value in an EL expression defined in the Advanced mode configuration field of a widget cannot be evaluated, a vague error message is generated in server.log, which does not help finding what widget is the culprit.
Error message examples:

[org.nuxeo.ecm.platform.forms.layout.service.WebLayoutManagerImpl] Could not get string value for 'false'