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/Platform/Hosted/Main.ts

@@ -23,6 +23,7 @@ import "../../Explorer/Tables/DataTable/DataTableBindingManager";
 import Explorer from "../../Explorer/Explorer";
 import { updateUserContext } from "../../UserContext";
 import { configContext } from "../../ConfigContext";
+import { getErrorMessage } from "../../Common/ErrorHandlingUtils";
 
 export default class Main {
   private static _databaseAccountId: string;
@@ -245,7 +246,7 @@ export default class Main {
         );
       },
       (error: any) => {
-        deferred.reject(`Failed to generate encrypted token: ${error.message}`);
+        deferred.reject(`Failed to generate encrypted token: ${getErrorMessage(error)}`);
       }
     );