message: Improve error for non-supported slot operations
diff: @@ -7,6 +7,7 @@ import { WebSiteManagementClient } from 'azure-arm-website';
  import { AppServicePlan, FunctionEnvelopeCollection, FunctionSecrets, HostNameSslState, Site, SiteConfigResource, SiteLogsConfig, SiteSourceControl, SourceControlCollection, StringDictionary, User, WebAppInstanceCollection } from 'azure-arm-website/lib/models';
  import { createAzureClient, ISubscriptionRoot, parseError } from 'vscode-azureextensionui';
  import { FunctionEnvelope } from 'vscode-azurekudu/lib/models';
  +import { localize } from './localize';
  import { nonNullProp, nonNullValue } from './utils/nonNull';
  /**
  @@ -43,6 +44,7 @@ export class SiteClient {
  public readonly gitUrl: string | undefined;
  private readonly _subscription: ISubscriptionRoot;
  + private _funcPreviewSlotError: Error = new Error(localize('functionsSlotPreview', 'This operation is not supported for slots, which are still in preview.'));
  constructor(site: Site, subscription: ISubscriptionRoot) {
  let matches: RegExpMatchArray | null = nonNullProp(site, 'serverFarmId').match(/\/subscriptions\/(.*)\/resourceGroups\/(.*)\/providers\/Microsoft.Web\/serverfarms\/(.*)/);
  @@ -173,8 +175,7 @@ export class SiteClient {
  public async listFunctions(): Promise<FunctionEnvelopeCollection> {
  if (this.slotName) {
  - // Functions support for slots is still in preview and doesn't support this yet
  - throw new Error('Method not implemented.');
  + throw this._funcPreviewSlotError;
  } else {
  return await this._client.webApps.listFunctions(this.resourceGroup, this.siteName);
  }
  @@ -182,8 +183,7 @@ export class SiteClient {
  public async listFunctionsNext(nextPageLink: string): Promise<FunctionEnvelopeCollection> {
  if (this.slotName) {
  - // Functions support for slots is still in preview and doesn't support this yet
  - throw new Error('Method not implemented.');
  + throw this._funcPreviewSlotError;
  } else {
  return await this._client.webApps.listFunctionsNext(nextPageLink);
  }
  @@ -191,8 +191,7 @@ export class SiteClient {
  public async getFunction(functionName: string): Promise<FunctionEnvelope> {
  if (this.slotName) {
  - // Functions support for slots is still in preview and doesn't support this yet
  - throw new Error('Method not implemented.');
  + throw this._funcPreviewSlotError;
  } else {
  return await this._client.webApps.getFunction(this.resourceGroup, this.siteName, functionName);
  }
  @@ -200,8 +199,7 @@ export class SiteClient {
  public async deleteFunction(functionName: string): Promise<void> {
  if (this.slotName) {
  - // Functions support for slots is still in preview and doesn't support this yet
  - throw new Error('Method not implemented.');
  + throw this._funcPreviewSlotError;
  } else {
  return await this._client.webApps.deleteFunction(this.resourceGroup, this.siteName, functionName);
  }
  