[octavia-dashboard] API validation errors are not displayed in the UI

[Gregory Thiemonge]

In octavia-dashboard master (6c9d092c298e5f6043052e0f2b9632b701dc4c1f)

When setting a field to an incorrect value (for instance '3000' in allowed cidrs), the request is sent to the Octavia API but the error is masked to the end user.

The dashboard only shows 'Error: Unable to create listener.'

While the Octavia API replies: 'Client-side error: Invalid input for field/attribute allowed_cidrs. Value: '['3000']'. Value should be IPv4 or IPv6 CIDR format'

The exceptions are caught by the dashboard but it only displays some generic error messages:

https://opendev.org/openstack/octavia-dashboard/src/branch/master/octavia_dashboard/static/app/core/openstack-service-api/lbaasv2.service.js#L227-L230

The dashboard should display (Error: Unable to create listener. Invalid input for field/attribute allowed_cidrs. Value: '['3000']'. Value should be IPv4 or IPv6 CIDR format)