HTTP response errors are not reported to user

[achannarasappa]

Describe the bug
When there is an unexpected response with making the HTTP request retrieve quotes, the UI shows blank content with no indication as to what the problem is.

To Reproduce
Steps to reproduce the behavior:

1. Start ticker with ticker -w AAPL --proxy "http://localhost:9999" (assuming no proxy exists at this port/host)
2. See empty UI with no error message

Expected behavior
UI to be populated with data or an error message indicating what the problem is

Proposed solution
When there is a HTTP error response, display message in UI or exit application with error

See #59 for more information and workaround