Merge pull request #37 from prabinv/23810_Handle500Errors

Added HTTP Response Interceptor - PR # 2 for TP 23810
29204: Adding tests to vnCart saveCart fn
* Removed session tests related to firebase prototype for SiteBuilder.
* Remove vn-session code and files from toolbox.
* Cleaned up index and main.js controller.
* Added a VnApiMockLibrary
* Added TestUtils
* Added test for the promise that returns data
* Add Errors and Warnings to the mocked response.
* Add tests for warnings and serviceErrors.
* Added reset fn to the vnCart service
* Added initialization test for two state properties after the cart has been inited
Code Review Suggestions:
* Be consistent with module injection in beforeEach
* Use vnApiEndpoint.getApiUrl() to drive the mock response set up
Merge pull request #39 from volusion-angular/29204-add-cart-test

29204
Added the vnErrorModalService to show the server error modal