Provide a custom error message when CORS fails

[hackaugusto]

Is your feature request related to a problem? Please describe.

When communication to the ethereum client fails because of CORS the webui presents a generic error message Connection failure

Describe the solution you'd like

Provide a descriptive message for the problem with a solution (e.g. "Communication with the Ethereum node was blocked because of the security policy, please enable communication to the ethereum node by executing the node with --rpccorsdomain {domain}")

Describe alternatives you've considered

Additional context