Fix error handling in DE (#294)

- Replaced `JSON.stringify(error)` with `error.message`
- Created `ErrorHandlingUtils` and moved all error logging actions in there

src/GitHub/GitHubContentProvider.ts

@@ -423,7 +423,7 @@ export class GitHubContentProvider implements IContentProvider {
       request: {},
       status: error.errno,
       response: error,
-      responseText: JSON.stringify(error),
+      responseText: error.message,
       responseType: "json"
     };
   }