Clean up frontend residue code that shows incorrect error messages

[kjw142857]

Currently, handleResponseError in the RequestsSaga checks if the response is null and if so, shows a message that the user is unable to reach the BE servers. However, the response handled defaults to null as the request handler does not forward non-200 responses, and so this message is always shown regardless of the actual error (e.g. unauthorised access, file not found)
This may cause users to misunderstand what the actual issue is

(all errors cause null to be forwarded)

(the forwarded null response causes the default incorrect server message to show)

[RichDom2185]

Somewhat related to #2574.