Custom error response. #1278

Is your feature request related to a problem? Please describe.
I'm unable to add simple web page informing about service and it's purpose.

Describe the solution you'd like
I't would be best to provide ability to override response when handshake not in place. (no connection upgrade)
I would suggest adding Exception argument to generateHttpResponseDueToError method and:

Either make generateHttpResponseDueToError protected or invoke from it like this to get error code page:

    protected String getErrorCodeDescriptionPage(int errorCode, Exception exception) {
    }

Describe alternatives you've considered
I tried to approach this by using refection to tinker with generateHttpResponseDueToError metod in WebSocketImpl class with no working results.

Additional context
None