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/Controls/Settings/SettingsComponent.tsx

@@ -50,6 +50,7 @@ import {
   getMongoDBCollectionIndexTransformationProgress,
   readMongoDBCollectionThroughRP
 } from "../../../Common/dataAccess/readMongoDBCollection";
+import { getErrorMessage } from "../../../Common/ErrorHandlingUtils";
 
 interface SettingsV2TabInfo {
   tab: SettingsV2TabTypes;
@@ -437,7 +438,7 @@ export class SettingsComponent extends React.Component<SettingsComponentProps, S
               defaultExperience: this.container.defaultExperience(),
               dataExplorerArea: Constants.Areas.Tab,
               tabTitle: this.props.settingsTab.tabTitle(),
-              error: error.message
+              error: getErrorMessage(error)
             },
             startKey
           );