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/Explorer/Tabs/SettingsTab.ts

@@ -22,6 +22,7 @@ import { CommandButtonComponentProps } from "../Controls/CommandButton/CommandBu
 import { userContext } from "../../UserContext";
 import { updateOfferThroughputBeyondLimit } from "../../Common/dataAccess/updateOfferThroughputBeyondLimit";
 import { configContext, Platform } from "../../ConfigContext";
+import { getErrorMessage } from "../../Common/ErrorHandlingUtils";
 
 const ttlWarning: string = `
 The system will automatically delete items based on the TTL value (in seconds) you provide, without needing a delete operation explicitly issued by a client application. 
@@ -1174,7 +1175,7 @@ export default class SettingsTab extends TabsBase implements ViewModels.WaitsFor
           defaultExperience: this.container.defaultExperience(),
           dataExplorerArea: Constants.Areas.Tab,
           tabTitle: this.tabTitle(),
-          error: error.message
+          error: getErrorMessage(error)
         },
         startKey
       );