refactor error handling part 1 (#307)

- created `getErrorMessage` function which takes in an error string or any type of error object and returns the correct error message
- replaced `error.message` with `getErrorMessage` since `error` could be a string in some cases
- merged sendNotificationForError.ts with ErrorHandlingUtils.ts
- some minor refactoring

In part 2, I will make the following changes:
 - Make `Logger.logError` function take an error message string instead of an error object. This will reduce some redundancy where the `getErrorMessage` function is being called twice (the error object passed by the caller is already an error message).
 - Update every `TelemetryProcessor.traceFailure` call to make sure we pass in an error message instead of an error object since we stringify the data we send.

src/GitHub/GitHubContentProvider.ts

@@ -9,6 +9,7 @@ import { NotebookUtil } from "../Explorer/Notebook/NotebookUtil";
 import { GitHubClient, IGitHubFile, IGitHubResponse } from "./GitHubClient";
 import * as GitHubUtils from "../Utils/GitHubUtils";
 import UrlUtility from "../Common/UrlUtility";
+import { getErrorMessage } from "../Common/ErrorHandlingUtils";
 
 export interface GitHubContentProviderParams {
   gitHubClient: GitHubClient;
@@ -423,7 +424,7 @@ export class GitHubContentProvider implements IContentProvider {
       request: {},
       status: error.errno,
       response: error,
-      responseText: error.message,
+      responseText: getErrorMessage(error),
       responseType: "json"
     };
   }