Sending a non-serializable modal context causes obtuse error message

[Blackbaud-ChristiSchneider]

I think the scope of this fix should just be to supply a useful error message such as "Non-serializable modal context not supported".

If I attempt to pass a function to the modal context, I get the error: DOMException: Failed to execute 'postMessage' on 'Window': function () { } could not be cloned.

Example code:

import { Component, Injectable } from '@angular/core';
import { SkyAppConfig } from '@skyux/config';
import { SkyAddinModalService } from '@blackbaud-internal/skyux-lib-addin-host';
import { AddinCloseModalArgs } from '@blackbaud-internal/sky-addin-host';
import { Router } from '@angular/router';

@Component({
  selector: 'example',
  templateUrl: './example.component.html'
})
@Injectable()
export class ExampleComponent {

  constructor(
    private skyAppConfig: SkyAppConfig,
    private skyAddinModalService: SkyAddinModalService,
    private router: Router
  ) { }

  public openExampleModal() {

    this.skyAddinModalService.openModal({
      id: 'Example Modal',
      url:
        `https://renxt.blackbaud.com/addins/addconstituent?envid=${this.skyAppConfig.runtime.params.get('envid')}&internalmodal=true`,
      modalMetadata: {
        context: {
          duplicateSelectedHandler: function () { }
        }
      }
    })
    .then((args: AddinCloseModalArgs) => {
      console.log(JSON.stringify(args));
    });
  }
}