API Fetch and XML Parse being thrown in pairs, instead of preventing further exceptions

[cohaolain]

Issue

Currently, if an API Fetch issue occurs, it will be followed by an XML Parse error. We shouldn't even try to parse an invalid API response (which would then just be NoneType.

API Fetch Error HTTPConnectionPool(host='api.irishrail.ie', port=80): Max retries exceeded with url: /realtime/realtime.asmx/getCurrentTrainsXML (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f74e1fe8520>: Failed to establish a new connection: [Errno 110] Connection timed out'))
XML Parse Error a bytes-like object is required, not 'NoneType'
API Fetch Error HTTPConnectionPool(host='api.irishrail.ie', port=80): Max retries exceeded with url: /realtime/realtime.asmx/getCurrentTrainsXML (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f74e1fe8310>: Failed to establish a new connection: [Errno 110] Connection timed out'))
XML Parse Error a bytes-like object is required, not 'NoneType'
API Fetch Error HTTPConnectionPool(host='api.irishrail.ie', port=80): Max retries exceeded with url: /realtime/realtime.asmx/getCurrentTrainsXML (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f74e20c6490>: Failed to establish a new connection: [Errno 110] Connection timed out'))
XML Parse Error a bytes-like object is required, not 'NoneType'

Proposed Fix

Ensure failed API calls don't have their responses parsed to begin with.